Transform into a full D-BUS service with Polkit support

Primarily we convert the service into a thread safe one that isn't reliant
on signaling for control flow, eliminating data race conditions. We also
enable interleaving by separating game mode pivoting from explicit client
registration.

The static pid list is now converted into a dynamic list that is OOM safe
to store all registered clients (with a reasonable upper limit of 256 clients)
to better handle cases where LD_PRELOAD is used for a large process group.
Additionally we begin storing some metadata on the connected clients such
as their executable path, which will enable us to perform some basic
whitelisting in future.

The cpugovctl binary is now moved into the libexecdir as an explicit helper
of the D-BUS service, using the shared library to merge some code back into
the daemon. This saves having to execute a process to query the state of the
governors, as we don't need a privileged client to do this.

In order to sanely set the governors, we require that the binary is running
as euid 0, and execute this using `pkexec`. A PolKit policy definition is
provided which allows active/logged in users to execute this helper through
a path whitelist. As such we can convert the daemon into user-mode only, with
the privileged helper being dispatched exclusively via polkit. This removes
the need for a setuid helper or having a system mode daemon.

Lastly we clean up the codebase a bit to be consistent with modern C code
conventions, using pragmas where available. The library component still uses
the older ifdef approach to support older compilers, but the daemon portion
uses the directive to simplify intent and speed up compilation. Additionally
we move all comments to C style comments for consistency, instead of mixing
in C++ style single line comments, in order to establish a formal coding
style.

The net result is a more robust service which can be D-BUS activated when
clients need it, that can perform scaling automatically without harassing
the user with authentication popups.

Signed-off-by: Ikey Doherty <ikey@solus-project.com>

lib/gamemode_client.h

@@ -40,34 +40,39 @@ POSSIBILITY OF SUCH DAMAGE.
 
 char _client_error_string[512] = {};
 
-// Load libgamemode dynamically to dislodge us from most dependencies
-// This allows clients to link and/or use this regardless of runtime
-// See SDL2 for an example of the reasoning behind this in terms of
-// dynamic versioning as well
-int _libgamemode_loaded = 1;
+/**
+ * Load libgamemode dynamically to dislodge us from most dependencies.
+ * This allows clients to link and/or use this regardless of runtime.
+ * See SDL2 for an example of the reasoning behind this in terms of
+ * dynamic versioning as well.
+ */
+volatile int _libgamemode_loaded = 1;
 
-// Typedefs for the functions to load
+/* Typedefs for the functions to load */
 typedef int (*_gamemode_request_start)();
 typedef int (*_gamemode_request_end)();
 typedef const char *(*_gamemode_error_string)();
 
-// Storage for functors
+/* Storage for functors */
 _gamemode_request_start _REAL_gamemode_request_start = NULL;
 _gamemode_request_end _REAL_gamemode_request_end = NULL;
 _gamemode_error_string _REAL_gamemode_error_string = NULL;
 
-// Loads libgamemode and needed functions
-// returns 0 on success and -1 on failure
+/**
+ * Loads libgamemode and needed functions
+ *
+ * Returns 0 on success and -1 on failure
+ */
 __attribute__((always_inline)) inline int _load_libgamemode()
 {
-	// We start at 1, 0 is a success and -1 is a fail
+	/* We start at 1, 0 is a success and -1 is a fail */
 	if (_libgamemode_loaded != 1) {
 		return _libgamemode_loaded;
 	}
 
 	void *libgamemode = NULL;
 
-	// Try and load libgamemode
+	/* Try and load libgamemode */
 	libgamemode = dlopen("libgamemode.so", RTLD_NOW);
 	if (!libgamemode) {
 		snprintf(_client_error_string,
@@ -82,7 +87,7 @@ __attribute__((always_inline)) inline int _load_libgamemode()
 		_REAL_gamemode_error_string =
 		    (_gamemode_error_string)dlsym(libgamemode, "real_gamemode_error_string");
 
-		// Verify we have the functions we want
+		/* Verify we have the functions we want */
 		if (_REAL_gamemode_request_start && _REAL_gamemode_request_end &&
 		    _REAL_gamemode_error_string) {
 			_libgamemode_loaded = 0;
@@ -99,10 +104,12 @@ __attribute__((always_inline)) inline int _load_libgamemode()
 	return -1;
 }
 
-// Redirect to the real libgamemode
+/**
+ * Redirect to the real libgamemode
+ */
 __attribute__((always_inline)) inline const char *gamemode_error_string()
 {
-	// If we fail to load the system gamemode, return our error string
+	/* If we fail to load the system gamemode, return our error string */
 	if (_load_libgamemode() < 0) {
 		return _client_error_string;
 	}
@@ -110,9 +117,11 @@ __attribute__((always_inline)) inline const char *gamemode_error_string()
 	return _REAL_gamemode_error_string();
 }
 
-// Redirect to the real libgamemode
-// Allow automatically requesting game mode
-// Also prints errors as they happen
+/**
+ * Redirect to the real libgamemod
+ * Allow automatically requesting game mode
+ * Also prints errors as they happen.
+ */
 #ifdef GAMEMODE_AUTO
 __attribute__((constructor))
 #else
@@ -120,7 +129,7 @@ __attribute__((always_inline)) inline
 #endif
 int gamemode_request_start()
 {
-	// Need to load gamemode
+	/* Need to load gamemode */
 	if (_load_libgamemode() < 0) {
 #ifdef GAMEMODE_AUTO
 		fprintf(stderr, "gamemodeauto: %s\n", gamemode_error_string());
@@ -138,7 +147,7 @@ int gamemode_request_start()
 	return 0;
 }
 
-// Redirect to the real libgamemode
+/* Redirect to the real libgamemode */
 #ifdef GAMEMODE_AUTO
 __attribute__((destructor))
 #else
@@ -146,7 +155,7 @@ __attribute__((always_inline)) inline
 #endif
 int gamemode_request_end()
 {
-	// Need to load gamemode
+	/* Need to load gamemode */
 	if (_load_libgamemode() < 0) {
 #ifdef GAMEMODE_AUTO
 		fprintf(stderr, "gamemodeauto: %s\n", gamemode_error_string());