pgsql: Fix dumping role comments when using --no-role-passwords

Fix dumping role comments when using --no-role-passwords

Commit 9a83d56b38c added support for allowing pg_dumpall to dump
roles without including passwords, which accidentally made dumps
omit COMMENTs on roles. This fixes it by using pg_authid to get
the comment.

Backpatch to all supported versions. Patch simultaneously written
independently by Álvaro and myself.

Author: Álvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Author: Daniel Gustafsson <daniel(at)yesql(dot)se>
Reported-by: Bartosz Chroł <bartosz(dot)chrol(at)handen(dot)pl>
Discussion: https://postgr.es/m/AS8P194MB1271CDA0ADCA7B75FCD8E767F7332@AS8P194MB1271.EURP194.PROD.OUTLOOK.COM
Discussion: https://postgr.es/m/CAEP4nAz9V4H41_4ESJd1Gf0v%3DdevkqO1%3Dpo91jUw-GJSx8Hxqg%40mail.gmail.com
Backpatch-through: v12

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d82cb467bbc292375283b40f5cc65868024c82b4

Modified Files
--------------
src/bin/pg_dump/pg_dumpall.c | 21 ++++++++++++---------
1 file changed, 12 insertions(+), 9 deletions(-)