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

src/backend/tcop/postgres.c

@@ -3933,9 +3933,32 @@ PostgresSingleUserMain(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 */
 	InitializeMaxBackends();
 
+	/*
+	 * Give preloaded libraries a chance to request additional shared memory.
+	 */
+	process_shmem_requests();
+
+	/*
+	 * Now that loadable modules have had their chance to request additional
+	 * shared memory, determine the value of any runtime-computed GUCs that
+	 * depend on the amount of shared memory required.
+	 */
+	InitializeShmemGUCs();
+
+	/*
+	 * Now that modules have been loaded, we can process any custom resource
+	 * managers specified in the wal_consistency_checking GUC.
+	 */
+	InitializeWalConsistencyChecking();
+
 	CreateSharedMemoryAndSemaphores();
 
 	/*