pg_dump: avoid unsafe function calls in getPolicies().

pg_dump: avoid unsafe function calls in getPolicies().

getPolicies() had the same disease I fixed in other places in
commit e3fcbbd62, i.e., it was calling pg_get_expr() for
expressions on tables that we don't necessarily have lock on.
To fix, restrict the query to only collect interesting rows,
rather than doing the filtering on the client side.

Like the previous patch, apply to HEAD only for now.

Discussion: https://postgr.es/m/2273648.1634764485@sss.pgh.pa.us
Discussion: https://postgr.es/m/7d7eb6128f40401d81b3b7a898b6b4de@W2012-02.nidsa.loc