gunixmounts: Return the last matching mount for same mount path
More mounts can have same mount path, but only the last one is accessible. Thus we should always return the last matching mount from g_unix_mount_at() and g_unix_mount_for(). This should also solve problems with g_file_trash() on automounted filesystems, which are caused by the recently added mount checks.
Closes: #1727 (closed)