[PATCHv2] Accept the timezone specifiers [+-]hh:mm and [+-]hh in addition to [+-]hhmm

Ok, here's a new attempt for the +hh(:mm) timezones, which should be semantically equivalent to the previous one. In particular, I tested the following corner cases:

  check_parse '+31/05/06 1980 01:02:03' '1980-05-06 01:02:03 +0000'
  check_parse '+1979/05/06/03 01:02:03' '2003-05-06 01:02:03 +0000'
  check_parse '+11:5:6 1999/04/01' '1999-03-31 18:06:00 +0000'
  check_parse '+11:73 Jun 15 10:00:00' bad

I'm not arguing that these results are "correct" or anything, I just observe that it was like that before, and so by conservatively not changing them, I shouldn't break anything...

(Ok, the last one _is_ a change from how it was in the before time,
 in the long long ago, but it's the same as it was with the last
 version of the patch.)

  // Marcus