Add some time-related helpers

Add some time-related helpers

RXGK_NOW(), a quick routine to get the current timestamp as an rxgkTime,
and secondsToRxgkTime for the more general scaling factor.

Change-Id: I0051b5c8e5ad61e35431d97454bf2741daba90cb
Reviewed-on: https://gerrit.openafs.org/10566
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>