lib: support the new pidfd based APIs

Try to make API requests using the new pidfd based APIs. If getting
the pidfds fails or if the remote (daemon) does not support the new
pidfd based D-Bus API, transparently fall back to the old API.

lib/meson.build

@@ -12,6 +12,7 @@ gamemode = shared_library(
         'client_impl.c',
     ],
     dependencies: [
+        link_lib_common,
         dep_dbus,
     ],
     install: true,