Use the recursive mutex functions for D-Bus only.

Use the recursive mutex functions for D-Bus only.

At least in D-Bus 1.2.16, there's a deadlock caused by locking an
already-locked mutex. When Qt is linked to libdbus-1 instead of
dynamically loading it, it maintains legacy compatibility by only
providing the non-recursive mutexes, which triggers this deadlock.

The recursive functions have been present since 0.93, so I guess the
non-recursive variants aren't tested.

Report: https://bugzilla.novell.com/show_bug.cgi?id=482749#c30