pgsql: Introduce pg_signal_autovacuum_worker.

Introduce pg_signal_autovacuum_worker.

Since commit 3a9b18b309, roles with privileges of pg_signal_backend
cannot signal autovacuum workers. Many users treated the ability
to signal autovacuum workers as a feature instead of a bug, so we
are reintroducing it via a new predefined role. Having privileges
of this new role, named pg_signal_autovacuum_worker, only permits
signaling autovacuum workers. It does not permit signaling other
types of superuser backends.

Bumps catversion.

Author: Kirill Reshke
Reviewed-by: Anthony Leung, Michael Paquier, Andrey Borodin
Discussion: https://postgr.es/m/CALdSSPhC4GGmbnugHfB9G0%3DfAxjCSug_-rmL9oUh0LTxsyBfsg%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ccd38024bc3c61e62af2097d408a670661713e68

Modified Files
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doc/src/sgml/func.sgml | 8 ++++++
doc/src/sgml/user-manag.sgml | 5 ++++
src/backend/storage/ipc/signalfuncs.c | 46 ++++++++++++++++++++++++++++-------
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_authid.dat | 5 ++++
5 files changed, 56 insertions(+), 10 deletions(-)