Avoid dirty afs tables on shutdown.

Avoid dirty afs tables on shutdown.

The server process must wait for afs because afs catches SIGINT/SIGTERM
and may thus not respond immediately to these SIGNALS.

Before reacting to the signal, afs might want to use the shared memory
area and the mmd mutex.  If the server process destroys this mutex
too early and afs tries to lock the shared memory area, the call to
mutex_lock() will fail and terminate the afs process. This leads to
dirty osl tables.

There's no such problem with the other children of the server process
(the command handlers) as these reset their SIGINT/SIGTERM handlers
to the default action, i.e.  these processes get killed immediately
by the above kill().