date: destroy "vary" data structure in main()

We free the "vary" data structure at the end of main() rather than in
show_date(). This is cleaner and it allows us to use "vary" expressions
in batch_convert(). Lastly, we can declare the vary pointer passed into
show_date() as const.
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.