ctdb-common: Move capture_socket functions to ctdb_socket.[ch]

ctdb-common: Move capture_socket functions to ctdb_socket.[ch]

The system_<os>.c files contain a lot of duplication, making
maintenance difficult.  These functions are being merged into
system_socket.c and system.c.

Bring across ctdb_sys_open_capture_socket(),
ctdb_sys_close_capture_socket() and ctdb_sys_read_tcp_packet().

Remove empty system_<os>.c files.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>