Fix bogus timetz_zone() results for DYNTZ abbreviations.

Fix bogus timetz_zone() results for DYNTZ abbreviations.

timetz_zone() delivered completely wrong answers if the zone was
specified by a dynamic TZ abbreviation, because it failed to account
for the difference between the POSIX conventions for field values in
struct pg_tm and the conventions used in PG-specific datetime code.

As a stopgap fix, just adjust the tm_year and tm_mon fields to match
PG conventions.  This is fixed in a different way in HEAD (388e71af8)
but I don't want to back-patch the change of reference point.

Discussion: https://postgr.es/m/CAJ7c6TOMG8zSNEZtCn5SPe+cCk3Lfxb71ZaQwT2F4T7PJ_t=KA@mail.gmail.com