This patch fixes some problems in date handling for atypical dates.

This patch fixes some problems in date handling for atypical dates.
Here is a summary:
Be more careful to check input string lengths as well as values
 when deciding whether a field is a year field.  Assume *anything* longer
 than 2 digits (if it isn't a special-case doy) is a valid year.
 This should fix the "Y1K" and "Y10K" problems
  pointed out by Massimo recently.
Check usage of BC to require a positive-valued year; before just used it
 to flip the sign of the year without checking. This led to problems
 near year zero.
Allow a 5 digit "concatenated date" of 2 digit year plus day of year.
Do 2->4 digit year correction for 6 and 5 digit "concatenated dates".
 Somehow forgot this originally. Guess not many folks use it...
Move common macros to dt.h.