FBSD: always close the rx socket when shutting down

FBSD: always close the rx socket when shutting down

The soclose(rx_socket) call is needed in order to restart AFS.
(Otherwise sobind() fails with EADDRINUSE.)
While here, use the defined constant SHUT_RDWR instead of
hardcoding '2' for soshutdown's second argument.

Change-Id: Iba7cac3aec79764783628da28b12253d496a7680
Reviewed-on: http://gerrit.openafs.org/2331
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>