Add a GUC to report whether data page checksums are enabled.

Backported from master. It was an oversight in the original data checksums
patch to not have a GUC like this.

src/backend/access/transam/xlog.c

@@ -3739,6 +3739,10 @@ ReadControlFile(void)
 				  " but the server was compiled without USE_FLOAT8_BYVAL."),
 				 errhint("It looks like you need to recompile or initdb.")));
 #endif
+
+	/* Make the initdb settings visible as GUC variables, too */
+	SetConfigOption("data_checksums", DataChecksumsEnabled() ? "yes" : "no",
+					PGC_INTERNAL, PGC_S_OVERRIDE);
 }
 
 void