Process shared_preload_libraries in single-user mode.

Without processing shared_preload_libraries, it's impossible to
recover if custom WAL resource managers are needed. It may also pose a
problem running VACUUM on a table with a custom AM, if the module
implementing the AM is expecting to be loaded by
shared_preload_libraries.

The reason this wasn't done before was just the general principle to
do fewer things in single-user mode. But it's easy enough to just set
shared_preload_libraries to empty, for the same effect.

Discussion: https://postgr.es/m/9decc18a42634f8a2f15c97a385a0f51a752f396.camel%40j-davis.com
Reviewed-by: Tom Lane, Andres Freund
Backpatch-through: 15
(cherry picked from commit 6c31ac0)

Author: jeff-davis

src/backend/tcop/postgres.c

@@ -5044,8 +5044,19 @@ PostgresMain(int argc, char *argv[],
 		/* read control file (error checking and contains config ) */
 		LocalProcessControlFile(false);
 
+		/*
+		 * process any libraries that should be preloaded at postmaster start
+		 */
+		process_shared_preload_libraries();
+
 		/* Initialize MaxBackends (if under postmaster, was done already) */
 		InitializeMaxBackends();
+
+		/*
+		 * Now that modules have been loaded, we can process any custom resource
+		 * managers specified in the wal_consistency_checking GUC.
+		 */
+		InitializeWalConsistencyChecking();
 	}
 
 	/* Early initialization */