Replace afs_ctime with strftime and friends

Replace afs_ctime with strftime and friends

Replace our local afs_ctime() hack with strftime and localtime_r,
which roken can provide for us if necessary. This avoids the
compatibility problems inherent with ctime_r, and removes another bit
of platform compatibility goo from libutil.

Change-Id: I18ed36cc4dce9aa354ad1398710ab7db83c814a2
Reviewed-on: http://gerrit.openafs.org/4426
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>