rx: Add rx_GetNetworkError

Add the function rx_GetNetworkError to rx, to allow callers to
retrieve some information about the last ICMP error received for a
connection's peer. This can be useful if a call on that connection was
recently ended due to ICMP errors.

Reviewed-on: http://gerrit.openafs.org/8451
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 399abf159b1418dea38daf166f9d55072042909c)

Change-Id: Icfb88e5ec6797d5dcec14e608d9b75e7983077fd
Reviewed-on: http://gerrit.openafs.org/10038
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>