pg_regress now needs to know that Windows hasn't got unix sockets,

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.