Bug#38435 - LONG Microseconds cause MySQL to fail a CAST to DATETIME or DATE

Parsing of optional microsecond part in datetime did not
fail gracefully when field width was larger than the allowed
six places.

Now handles up to the correct six places, and disregards
any extra digits without messing up what we've already got.

mysql-test/r/type_datetime.result:
  show graceful handling of overly long microsecond parts
  (correct truncation).
mysql-test/t/type_datetime.test:
  show graceful handling of overly long microsecond parts
  (correct truncation).
sql-common/my_time.c:
  Special case for time-parsing: for microsecond part,
  leading zeroes are actually meaningful! Also, don't
  break the entire date on more than the allowed six
  digits in microsecond part, just truncate the extra
  digits.