MFC: r279597 (and 259152,259219 as dependency)

Make calendar(1) usable again. rework the calendar parser so that it is
compatible with documented format:

Support includes surrounded by '"' or '<' '>'
Print warnings about bad syntax
Correctly navigate through include directories to find calendar files
Correctly support multiple includes

While here:

MFC: 262011 (by eadler)

calendar(1): don't segfault in invalid input

When the user supplies an invalid number of days provide a useful error message
instead of segfaulting.

Approved by: re (gjb)