pgsql: Fix pg_regress to print the correct postmaster address on Window

Fix pg_regress to print the correct postmaster address on Windows.

pg_regress reported "Unix socket" as the default location whenever
HAVE_UNIX_SOCKETS is defined.  However, that's not been accurate
on Windows since 8f3ec75de.  Update this logic to match what libpq
actually does now.

This is just cosmetic, but still it's potentially misleading.
Back-patch to v13 where 8f3ec75de came in.

Discussion: https://postgr.es/m/3894060.1646415641@sss.pgh.pa.us

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/a008c03dd722f3a9ea44b13ca2cc6066fd90ddc1

Modified Files
--------------
src/interfaces/libpq/fe-connect.c |  5 +++++
src/test/regress/pg_regress.c     | 10 ++++++++--
2 files changed, 13 insertions(+), 2 deletions(-)