fix the plm database tool

fix the plm database tool

(Re)loading the playlist uses the new mutex and
shm helpers and is hopefully a race-free implementation.

It works as follows:

	The plm init function (parent, server context) reserves
	a shared memory area (client data) as well as two unlocked
	mutexes. The first mutex serializes access for racing clients
	that want to replace the playlist. The second mutex serializes
	the access between parent and child (com_lpl()).

	com_lpl() loads the playlist from the client into a tmp buffer.
	It then creates a shm area (client_shm) and copies that buffer
	to the area.  Further, another mutex (shm) is created. This
	mutex is initially locked.

	Next, it grabs the client lock and the server lock, and updates
	client_data with the id of client_shm. Then the parent is
	woken up (SIGUSR1) and the server lock is released. com_lpl()
	finally goes to sleep by acquiring the shm lock.

	In this situation, only the parent may run. It grabs the
	server lock, reads client_data to attach client_shm and
	reloads the playlist with data from client_shm. When ready,
	the parent detaches client_shm, and drops the shm lock and
	the server lock.

	This wakes up com_lpl() which destroys client_shm, the
	shm mutex and the tmp buffer.