date: Add vary functionality.

We modify date to recognize the command line options -v and --varity
and to call the appropriate vary functions.
date: vary usage and man-page

Added -v/--varity command line argument to usage(). Also added examples
on how to use the feature.

The full description of the syntax was added to the date.x file.

All examples and descriptions were taken straight from the FreeBSD
man-page.