FBSD: clean up rx_socket teardown

FBSD: clean up rx_socket teardown

We had previously been waiting for the SO_ISDISCONNECTED flag
to show up in the so_state field, but the flags are not really
used for stateless protocols such as UDP, and that flag never
shows up.  Even with a full three-second wait, the rxk_Listener
sometimes still failed to fully terminate, preventing the reuse
of the rx port for a restarted afsd.
Copy from Solaris and loop until rxk_ListenerPid is zero, doing
bogus one-byte NetSends in the body of the loop.

Change-Id: I6a5ad4509549fa64c551af642ef5867602b182c0
Reviewed-on: http://gerrit.openafs.org/3391
Reviewed-by: Matt Benjamin <matt@linuxbox.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>