pgsql: Teach pg_dump to dump comments on RLS policy objects.

Teach pg_dump to dump comments on RLS policy objects.

This was unaccountably omitted in the original RLS patch.
The SQL syntax is basically the same as for comments on triggers,
so crib code from dumpTrigger().

Per report from Marc Munro.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/1581889298.18009.15.camel@bloodnok.com

Branch
------
REL9_5_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/d9ec8d1d7dc2bfaa7ae001c4319f6988a83dd486

Modified Files
--------------
src/bin/pg_dump/pg_dump.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)