ctdb-common: Move ctdb_sys_send_tcp() to ctdb_socket.[ch]

ctdb-common: Move ctdb_sys_send_tcp() 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 tcp_checksum(), renamed to ip_checksum().
uint16_checksum() becomes static.

Use the BSD struct tcphdr field names for portability.  See the
comment in the code for more details about how we get this to compile
on older glibc versions.

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