Fix dblink_connect() so that it verifies that a password is supplied in the

Fix dblink_connect() so that it verifies that a password is supplied in the
conninfo string *before* trying to connect to the remote server, not after.
As pointed out by Marko Kreen, in certain not-very-plausible situations
this could result in sending a password from the postgres user's .pgpass file,
or other places that non-superusers shouldn't have access to, to an
untrustworthy remote server.  The cleanest fix seems to be to expose libpq's
conninfo-string-parsing code so that dblink can check for a password option
without duplicating the parsing logic.

Joe Conway, with a little cleanup by Tom Lane