Move finding, opening and managment of the KMS file descriptor to
MetaMonitorManagerKms. This means that the monitor manager creation can
now fail, both if more than one GPU with connectors is discovered, or
if finding or opening the primary GPU fails.
https://bugzilla.gnome.org/show_bug.cgi?id=785381
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
