Clean up the ancient decision to show only two fractional-seconds digits

Clean up the ancient decision to show only two fractional-seconds digits
in "postgres_verbose" intervalstyle, and the equally arbitrary decision to
show at least two fractional-seconds digits in most other datetime display
styles.  This results in some minor changes in the expected regression test
outputs.

Also, coalesce a lot of repetitive code in datetime.c into subroutines,
for clarity and ease of maintenance.  In particular this roughly halves
the number of #ifdef HAVE_INT64_TIMESTAMP segments.

Ron Mayer, with some additional kibitzing from Tom Lane