kauth: fix clock skew detection

kauth: fix clock skew detection

Commit 5b3c1042969daec38ccb260e61d665eda0c713ea changed/removed some
uses of abs() on unsigned time values. While the previous use of abs()
was indeed incorrect, the result wasn't necessarily much better, even
though it built with recent compilers, since it only checked for skew
in one direction.

Define and use a  macro to correctly evaluate the conditionals in 64-bit
precision, avoiding C's integer promotion rules which prefer unsigned types
(Date) to signed types of the same width (time_t on 32-bit systems).

Reviewed-on: http://gerrit.openafs.org/11850
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 810f0ccd0354dac30af024ca7b5acf3ebabf5f4b)

Change-Id: I29337e1ecd410fcf7733408287930c50c055ff90
Reviewed-on: http://gerrit.openafs.org/11863
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>