main: initialize gsm-system before acquring name

gsm-systemd makes synchronous calls to logind.  These
calls invoke a main loop.  If we've already acquired a name
on the session bus when the main loop is running, then we'll
service that bus name too early.

This commit initializes gsm-system before acquiring a name to
avoid that clash.

https://bugzilla.gnome.org/show_bug.cgi?id=687804