Add a tunable "AllowClientDBAttach" with default value 1.

Add a tunable "AllowClientDBAttach" with default value 1.

When set to 0, clients will not be able to attach to databases
via the db_attach control. This might can be useful for maintenance
where ctdb should be kept running but clients should not be able
to modify databases.

(cherry picked from commit ddfeecda87955b4e46777599f678e6926d37f4c4)

Conflicts:
	include/ctdb_private.h
	server/ctdb_ltdb_server.c
	server/ctdb_tunables.c