pgsql: pg_regress now needs to know that Windows hasn't got unix

Log Message:
-----------
pg_regress now needs to know that Windows hasn't got unix sockets,
per Andrew Dunstan.  Also, don't override the user's value of PGHOST
in the 'make installcheck' case.  I think the latter was an ill-considered
workaround for the Windows code back when libpq didn't properly default
to localhost on Unix-socket-less platforms.

Modified Files:
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    pgsql/src/test/regress:
        pg_regress.sh (r1.52 -> r1.53)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/pg_regress.sh.diff?r1=1.52&r2=1.53)