Rename g_unix_mount_monitor_new() to _get()

Rename g_unix_mount_monitor_new() to _get()

This is a singleton, but we have a function called _new() to get it.
What's worse is that the documentation makes no mention of this, and
actually specifically says that a new monitor will be created each time.

https://bugzilla.gnome.org/show_bug.cgi?id=742599