ctdb-common: Move ctdb_get_peer_pid() to system.[ch]

ctdb-common: Move ctdb_get_peer_pid() to system.[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.

This function doesn't need ctdb_sock_addr so put it with general
system utilities.

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