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

Log Message:
-----------
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

Modified Files:
--------------
pgsql/src/backend/utils/adt:
datetime.c (r1.198 -> r1.199)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/datetime.c?r1=1.198&r2=1.199)
pgsql/src/test/regress/expected:
horology.out (r1.68 -> r1.69)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/horology.out?r1=1.68&r2=1.69)
interval.out (r1.25 -> r1.26)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/interval.out?r1=1.25&r2=1.26)
timestamp.out (r1.38 -> r1.39)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/timestamp.out?r1=1.38&r2=1.39)
timestamptz.out (r1.32 -> r1.33)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/timestamptz.out?r1=1.32&r2=1.33)
pgsql/src/test/regress/sql:
interval.sql (r1.17 -> r1.18)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/interval.sql?r1=1.17&r2=1.18)