Clean up a couple of weird corner cases in interval parsing: make -yyyy-mm be

Clean up a couple of weird corner cases in interval parsing: make -yyyy-mm be
interpreted as expected (the sign should affect months too), and get rid of
hard-wired assumption that unmarked signed values must be hours (if integers)
or seconds (if floats).  The former was just a bug in my previous patch,
while the latter may have made sense at one time but seems illogical now
that we support determination of the units from typmod information.
Ron Mayer and myself.