vos status: report human-readable last*Time

vos status: report human-readable last*Time

Make 'vos status' print the lastReceiveTime and lastSendTime in a
human-readable format, instead of just printing the number of seconds
since the epoch as an integer. As suggested by Jeffrey Altman.

Change-Id: I1348a45078e49ff631cb46323767f1c384a72084
Reviewed-on: http://gerrit.openafs.org/2338
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>