threads: misc mutex checks/speedup

• assert if a recursively locked mutex is used in a condition wait (not implemented)
• avoid a useless vlc_mutex_held() for recursive locks (which are usually not recursed)
• explicitly store 0 (not recursive) and 1 in the recursion counter on init
• do not store the owner in non-recursive mutex in release builds, it should never be used and avoids a costly call at each lock