afpd: distribute SIGHUP from parent afpd to children and force share reload

Signed-off-by: Ralph Boehme <rb@sernet.de>
netatalk: refresh Zeroconf registration when receiving SIGHUP

Signed-off-by: Ralph Boehme <rb@sernet.de>
libatalk:config: deleting volumes

Removing a volume from config which happened to be the first in the
list of loaded volumes would result in the global volume handle be set
to NULL.

Signed-off-by: Ralph Boehme <rb@sernet.de>