Make session saving work a bit better

Previously, the restart command hardcoded "mutter" as the binary name.
This commit changes it to use g_get_prgname() which has a better chance
of being correct (and it does fix session saving for gnome-shell).

Now that mutter is a library, it might be more correct (but also
much higher overhead) to add api for setting these things from
the outside.

https://bugzilla.gnome.org/show_bug.cgi?id=648828