Merge from vendor branch TNF:

Merge from vendor branch TNF:
Import p5-Date-Simple from pkgsrc-wip.  Packaged by George Michaelson.

Dates are complex enough without times and timezones.  This module may
be used to create simple date objects.  It handles:

Validation:
    Reject 1999-02-29 but accept 2000-02-29.
Interval arithmetic:
    How many days were between two given dates?  What date comes N days
    after today?
Day-of-week calculation:
    What day of the week is a given date?

It does NOT deal with hours, minutes, seconds, and time zones.