rx: Save errno in LWP rxi_Sendmsg

Much of this code examines errno or WSAGetLastError to determine what
to do. However, some other operations between the actual sendmsg call
and code that examines errno may modify errno. So, save the value of
errno to ensure errno reflects the actual error we got from sendmsg;
this also slightly simplifies some of the logic.

Reviewed-on: http://gerrit.openafs.org/8110
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 347908cd0031c558c266ec412aa49200ae2b3204)

Change-Id: Ia0134acb2ae5f9e47fb82f36bd0889987803d897
Reviewed-on: http://gerrit.openafs.org/9496
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>