helpers: add helpers to automatically close fds

Add a inline helper function and a helper macro to be able to
automatically close file descriptors. Does nothing if the argument
is NULL or the pointed to integer is < 0.

daemon/meson.build

@@ -30,6 +30,7 @@ daemon_sources = [
     'daemonize.c',
     'dbus_messaging.c',
     'daemon_config.c',
+    'helpers.c',
 ]
 
 gamemoded_includes = libgamemode_includes