rx: Add rx_GetNetworkError

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.

Change-Id: I7c507185f6a4ab986c0119f67cdaa4e50e447175
Reviewed-on: http://gerrit.openafs.org/8451
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>