metadata: GDBusServer new-connection signal needs a return value

Turns out this is kind of important and has been working mostly by
luck until now.  When compiled with gcc's -fstack-protector-strong,
we ended up returning 0, which in this case means that the connection
isn't interesting, so it stops processing messages on it.

https://bugzilla.gnome.org/show_bug.cgi?id=706470