Use -errno or WSAGetLastError() as return value from rxi_Sendmsg()

Use -errno or WSAGetLastError() as return value from rxi_Sendmsg()

This patch causes the pthread and lwp implementations of rxi_Sendmsg()
to use -errno or WSAGetLastError() as the return value if it is
positive.  This is required in order to communicate more meaningful
error conditions to rxi_SendPacket[List], which should "down" a host
immediately when it observes ENETUNREACH.

Jeff Altman <jaltman@openafs.org> supplied the logic for the
AFS_NT40_ENV case; I was not able to test this (I do not own a
Windows license) and took him on his word.

Reviewed-on: http://gerrit.openafs.org/58
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Adam Megacz <megacz@gmail.com>
Tested-by: Adam Megacz <megacz@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>