In rxi_SendPacket[List], down hosts if ENETUNREACH or equivalent

In rxi_SendPacket[List], down hosts if ENETUNREACH or equivalent

This patch causes rxi_SendPacket[List] to "down" a host if
rxi_Sendmsg() returns -ENETUNREACH (or equivalent).  Previous behavior
only did this check if defined(KERNEL) becauase non-defined(KERNEL)
implementations did not return meaningful error codes from
rxi_Sendmsg().

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/59
Reviewed-by: Adam Megacz <megacz@gmail.com>
Tested-by: Adam Megacz <megacz@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>