common: Fix documentation for hotplug script

The code at the bottom says we'll ignore further configuration if
"1" is returned, but the top said if "0" is returned.

Make both say "1".
common: Check the exit status of hotplug scripts correctly

Instead of comparing the shell's exit code by hand, use
g_spawn_check_exit_status() to get the script's exit code.

https://bugzilla.gnome.org/show_bug.cgi?id=710791