validate_info_seg: ensure 4-digit year is used in info segment header line date

[Gary Dixon]

Changes to Multics info segment made after 2000 need to better distinguish the year of change in the info segment header line, so there will be no roll-over of dates around the year 2070. The current rule is to always use a 4-digit year in the info segment header line.

85-10-10 validate_info_seg, vis

would become:

1985-10-10 validate_info_seg, vis

Dates not in calendar-date format (with year appearing first, followed my month and day) would be diagnosed as errors, recommending replacement with a 4-digit year in a calendar-date format. Thus:

02/06/85 check_info_segs, cis

would be diagnosed as an error, to be replaced by:

1985-02-06 check_info_segs, cis