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

src/backend/utils/misc/guc_tables.c

@@ -2168,6 +2168,17 @@ struct config_int ConfigureNamesInt[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"reserved_connections", PGC_POSTMASTER, CONN_AUTH_SETTINGS,
+			gettext_noop("Sets the number of connection slots reserved for roles "
+						 "with privileges of pg_use_reserved_connections."),
+			NULL
+		},
+		&ReservedConnections,
+		0, 0, MAX_BACKENDS,
+		NULL, NULL, NULL
+	},
+
 	{
 		{"min_dynamic_shared_memory", PGC_POSTMASTER, RESOURCES_MEM,
 			gettext_noop("Amount of dynamic shared memory reserved at startup."),

src/backend/utils/misc/postgresql.conf.sample

@@ -63,6 +63,7 @@
 					# (change requires restart)
 #port = 5432				# (change requires restart)
 #max_connections = 100			# (change requires restart)
+#reserved_connections = 0		# (change requires restart)
 #superuser_reserved_connections = 3	# (change requires restart)
 #unix_socket_directories = '/tmp'	# comma-separated list of directories
 					# (change requires restart)