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>

daemon/gamemode.h

@@ -28,16 +28,48 @@ ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 POSSIBILITY OF SUCH DAMAGE.
 
  */
-#ifndef _GAME_MODE_GAMEMODE_H_
-#define _GAME_MODE_GAMEMODE_H_
 
-// Initialise or terminate the game mode system
-void init_game_mode();
-void term_game_mode();
+#pragma once
 
-// Add or remove games to the tracker
-// Tracker will automatically start and stop game mode as appropriate
-void register_game(int pid);
-void unregister_game(int pid);
+#include <stdbool.h>
+#include <sys/types.h>
 
-#endif // _GAME_MODE_GAMEMODE_H_
+/**
+ * Opaque context
+ */
+typedef struct GameModeContext GameModeContext;
+
+/**
+ * Return the singleton instance
+ */
+GameModeContext *game_mode_context_instance(void);
+
+/**
+ * Initialise the GameModeContext
+ *
+ * This is performed in a thread-safe fashion.
+ */
+void game_mode_context_init(GameModeContext *self);
+
+/**
+ * Destroy the previously initialised GameModeContext.
+ *
+ * This is performed in a thread safe fashion.
+ */
+void game_mode_context_destroy(GameModeContext *self);
+
+/**
+ * Register a new game client with the context
+ *
+ * @param pid Process ID for the remote client
+ * @returns True if the new client could be registered
+ */
+bool game_mode_context_register(GameModeContext *self, pid_t pid);
+
+/**
+ * Unregister an existing remote game client from the context
+ *
+ * @param pid Process ID for the remote client
+ * @returns True if the client was removed, and existed.
+ */
+bool game_mode_context_unregister(GameModeContext *self, pid_t pid);