Fix: libcrmcommon: allow ISO 8601 strings of form "<date>T<time> <offset>"

Previously, the date parser would look for the time portion of the string by
searching for the first space, or if none, the first 'T'. However that would
incorrectly find the offset for the common form "<date>T<time> <offset>".

The new search does not perfectly detect all invalid formats (e.g. extra digits
at end of a YYYYMMDD will simply be ignored), but it should be reasonable.