pgsql: Simplify a couple of pg_dump and psql \d queries about index

Log Message:
-----------
Simplify a couple of pg_dump and psql \d queries about index constraints
by joining to pg_constraint.conindid, instead of the former technique of
joining indirectly through pg_depend.  This is much more straightforward
and probably faster as well.  I had originally desisted from changing these
queries when conindid was added because I was worried about losing
performance, but if we join on conrelid as well as conindid then the index
on conrelid can be used when pg_constraint is large.

Modified Files:
--------------
    pgsql/src/bin/pg_dump:
        pg_dump.c (r1.577 -> r1.578)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_dump.c?r1=1.577&r2=1.578)
    pgsql/src/bin/psql:
        describe.c (r1.239 -> r1.240)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/describe.c?r1=1.239&r2=1.240)