pgsql: Fix dblink_connect() so that it verifies that a password is

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

Modified Files:
--------------
    pgsql/contrib/dblink:
        dblink.c (r1.74 -> r1.75)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/dblink.c?r1=1.74&r2=1.75)
    pgsql/doc/src/sgml:
        dblink.sgml (r1.4 -> r1.5)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/dblink.sgml?r1=1.4&r2=1.5)
        libpq.sgml (r1.263 -> r1.264)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/libpq.sgml?r1=1.263&r2=1.264)
    pgsql/src/interfaces/libpq:
        exports.txt (r1.21 -> r1.22)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/exports.txt?r1=1.21&r2=1.22)
        fe-connect.c (r1.360 -> r1.361)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-connect.c?r1=1.360&r2=1.361)
        libpq-fe.h (r1.143 -> r1.144)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/libpq-fe.h?r1=1.143&r2=1.144)