Refactor the birthdate parsing logic, add unit test

The birthdate parsing is now a lot more robust and will recognize all kinds
of variants/formats and use range checks to disambiguate provided dates and
even support partial dates (e.g. year only, year/month and month/year).

The tests should all pass regardless of what the sysop has set for their
system date format in SCFG.

I had considered using sscanf() for the parsing logic but ended up going with
strtoul() only and that seems to be working well (as reflected by the passing
test cases).