Fix SIGHUP config reloading

Fix SIGHUP config reloading

Add SIGHUP handler to master 'netatalk' process and let it distribute
the signal to it's childs, eg afpd.

Add function afp_config_free() which releases ressources allocated
by afp_config_parse().
Additionally fix configinit()/configfree() which deal with ressources
only used in the afpd fileserver (eg sockets, zeroconf registration).

Fix an error in the mdns unregister code where pthread_kill() was
used but that somehow killed the whole process not just the mdns
thread. Use pthread_cancel() instead which seems to work.

Fix any ressource leak reported by libumem.