lib/util/run_cmd: prevent zombies in samba_runcmd_send on timeout

lib/util/run_cmd: prevent zombies in samba_runcmd_send on timeout

Ensure the state desctructor calls tfork_destroy to reap the waiter and
worker processes. Otherwise we leave the waiter process as a zombie
behind us as we never call waitpid on it in case of a timeout
or talloc_free() from the caller.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=13062

Pair-programmed-with: Stefan Metzmacher <metze@samba.org>

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Ralph Boehme <slow@samba.org>