Apply quotes more consistently to GUC names in logs

Quotes are applied to GUCs in a very inconsistent way across the code
base, with a mix of double quotes or no quotes used.  This commit
removes double quotes around all the GUC names that are obviously
referred to as parameters with non-English words (use of underscore,
mixed case, etc).

This is the result of a discussion with Álvaro Herrera, Nathan Bossart,
Laurenz Albe, Peter Eisentraut, Tom Lane and Daniel Gustafsson.

Author: Peter Smith
Discussion: https://postgr.es/m/CAHut+Pv-kSN8SkxSdoHano_wPubqcg5789ejhCDZAcLFceBR-w@mail.gmail.com

src/backend/storage/buffer/localbuf.c

@@ -705,7 +705,7 @@ check_temp_buffers(int *newval, void **extra, GucSource source)
 	 */
 	if (source != PGC_S_TEST && NLocBuffer && NLocBuffer != *newval)
 	{
-		GUC_check_errdetail("\"temp_buffers\" cannot be changed after any temporary tables have been accessed in the session.");
+		GUC_check_errdetail("temp_buffers cannot be changed after any temporary tables have been accessed in the session.");
 		return false;
 	}
 	return true;

src/backend/storage/file/fd.c

@@ -3931,7 +3931,7 @@ check_debug_io_direct(char **newval, void **extra, GucSource source)
 
 	if (!SplitGUCList(rawstring, ',', &elemlist))
 	{
-		GUC_check_errdetail("invalid list syntax in parameter \"%s\"",
+		GUC_check_errdetail("invalid list syntax in parameter %s",
 							"debug_io_direct");
 		pfree(rawstring);
 		list_free(elemlist);

src/backend/storage/lmgr/predicate.c

@@ -1644,7 +1644,7 @@ GetSerializableTransactionSnapshot(Snapshot snapshot)
 		ereport(ERROR,
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 				 errmsg("cannot use serializable mode in a hot standby"),
-				 errdetail("\"default_transaction_isolation\" is set to \"serializable\"."),
+				 errdetail("default_transaction_isolation is set to \"serializable\"."),
 				 errhint("You can use \"SET default_transaction_isolation = 'repeatable read'\" to change the default.")));
 
 	/*