g_error() is the wrong thing to do when, for example, we can't find the
DRM device, since Mutter should just fail to start rather than reporting
a bug into automatic bug tracking systems. Rather than trying to decipher
which errors are "expected" and which not, just make all failure paths
in meta_launcher_new() return a GError out to the caller - which we make
exit(1).
https://bugzilla.gnome.org/show_bug.cgi?id=757311
This uses David Herrmann's new logind sessions interface to retrieve
fds for input devices, rather than using a custom setuid helper to do
the management. This vastly simplifies the interface.
This requires systemd v210, at least.
https://bugzilla.gnome.org/show_bug.cgi?id=724604
