Refactor datetime functions' timezone lookup code to reduce duplication.

Refactor datetime functions' timezone lookup code to reduce duplication.

We already had five copies of essentially the same logic, and an
upcoming patch introduces yet another use-case.  That's past my
threshold of pain, so introduce a common subroutine.  There's not
that much net code savings, but the chance of typos should go down.

Inspired by a patch from Przemysław Sztoch, but different in detail.

Discussion: https://postgr.es/m/01a84551-48dd-1359-bf7e-f6b0203a6bd0@sztoch.pl