Patch for mhical to support "BYDAY=-1SU"-type RRULEs.

Patch for mhical to support "BYDAY=-1SU"-type RRULEs.

Notes:

- Added dmlastday() function to dtime.c, to obtain last day of month.
  Minor modifications to other parts here to reuse code.

- Modified rrule_clock() in datetime.c to handle negative BYDAY
  correctly.

- Added a couple of tests.

- Mainly this is to scratch an itch since UK timezone history makes
  heavy use of BYDAY=-1SU-type RRULEs, and the resultant complaints
  are annoying.  I had a brief look at "properly" extending
  icalparse.y, but I kindof think any serious extension of mhical
  should really instead adopt libical?