GNU/Hurd support

GNU/Hurd support

CTDB has the following limitations on GNU Hurd:

- The pid of a peer is not get from the socket [1]. As a consequence, the peer
  process is not killed when releasing IP [2].

- Gratuitous arp are not yet supported [3]

- network interfaces are always considered present [4]

[1]: ctdb_get_peer_pid() in common/system_gnu.c
[2]: release_kill_clients() in server/ctdb_takeover.c
[3]: ctdb_sys_send_arp() in common/system_gnu.c
[4]: ctdb_sys_check_iface_exists() in common/system_gnu.c

(This used to be ctdb commit 00212e5c7dd229e7f8975a165d5ab8875d4917cc)