if input year for date_add() / date_sub() with INTERVAL is low enough for
calc_daynr() to possibly return incorrect results (calc_daynr() has no information
on whether the year is low because it was a two-digit year ('77) or because it
was a really low four-digit year (0077) and will indiscriminately try to turn the
value into a four-digit year by adding 1900 or 2000 respectively), the functions
will now throw NULL.


include/my_time.h:
  new define YY_MAGIC_BELOW: if year is below this threshold, magic kicks in in
  calc_daynr(). the idea is to convert two-digit years to four-digit ones, adding
  1900 to values >= YY_PART_YEAR or adding 2000 otherwise.
  current value of YY_MAGIC_BELOW derived from original code in calc_daynr()
mysql-test/r/func_time.result:
  test where 2 digit -> 4 digit year magic kicks in, and whether we throw NULL when
  it happens
mysql-test/t/func_time.test:
  test where 2 digit -> 4 digit year magic kicks in, and whether we throw NULL when
  it happens
sql-common/my_time.c:
  use new const YY_MAGIC_BELOW, apply 2-digit -> 4-digit magic only to years
  below this threshold.
sql/item_timefunc.cc:
  throw NULL when year in date_add() / date_sub() would be affected by
  2 digit -> 4 digit magic.