pgsql: Add new GUC reserved_connections.

Add new GUC reserved_connections.

This provides a way to reserve connection slots for non-superusers.
The slots reserved via the new GUC are available only to users who
have the new predefined role pg_use_reserved_connections.
superuser_reserved_connections remains as a final reserve in case
reserved_connections has been exhausted.

Patch by Nathan Bossart. Reviewed by Tushar Ahuja and by me.

Discussion: http://postgr.es/m/20230119194601.GA4105788@nathanxps13

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6e2775e4d4e47775f0d933e4a93c148024a3bc63

Modified Files
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doc/src/sgml/config.sgml | 39 +++++++++++++++++++++++++--
doc/src/sgml/user-manag.sgml | 5 ++++
src/backend/postmaster/postmaster.c | 29 +++++++++++++-------
src/backend/storage/lmgr/proc.c | 16 +++++++----
src/backend/utils/init/postinit.c | 31 +++++++++++++++------
src/backend/utils/misc/guc_tables.c | 11 ++++++++
src/backend/utils/misc/postgresql.conf.sample | 1 +
src/include/catalog/pg_authid.dat | 5 ++++
src/include/postmaster/postmaster.h | 1 +
src/include/storage/proc.h | 2 +-
10 files changed, 115 insertions(+), 25 deletions(-)