[interfacekit] parsedate() now complete

Shortly after I introduced the parsedate() functions to the Support 
Kit, they are now complete.

I will do the docs later on, and since we already have some doxygen 
style docs, I will do it using that format - if we need to convert it 
to XML, I am sure we can easily extend doxygen to produce that format, 
too (as it already does HTML).

Anyway, for your reading pleasure, a short list of what I changed wrt 
the R5 implementation:
- "last tuesday" (and friends) is working correctly; unlike "next 
tuesday", it didn't work in R5
- "next/last x minutes/seconds/hours" is now also accepted as a 'T' (a 
time unit), was invalid before
- sets the PARSEDATE_MINUTE_RELATIVE_TIME if the parsed time would 
change with every minute passed (i.e. Tracker right now only redo the 
query daily, even if you ask for files changed in the past 5 minutes)
- now sets PARSEDATE_INVALID_DATE if the date could not be parsed - to 
be able to distinguish between unknown and invalid dates (the former 
could be fixed by adding more formats)
- many time zone specifications are still missing, I only added "GMT"

Like the original, it will ignore wrong weekday settings (like "Monday, 
2003/08/26", even if it's a tuesday).

Also, I've done some speed tests, in order to see if my solution is 
appropriate, every conversion was carried out 500000 times (OpenBeOS 
vs. R5):
strings like "next tuesday":
        9.5s vs. 13s
absolute date "1976-12-15":
        6.7s vs. 11.2s
empty string "":
        0.06s vs. 7.1s
NULL string:
        0.04s vs. crash
unparseable string, like "Gurkensalat":
        1.5s vs. 11.1s

Looks good enough to me. Of course, adding more time zones could cause 
slightly more overhead in the case of the unparseable string.

Adios...
   Axel.

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