Put back allow_system_table_mods check in heap_create().

This reverts commit a475c60367.

Erik Rijkers reported back in January 2013 that after the patch, if you do
"pg_dump -t myschema.mytable" to dump a single table, and restore that in
a database where myschema does not exist, the table is silently created in
pg_catalog instead. That is because pg_dump uses
"SET search_path=myschema, pg_catalog" to set schema the table is created
in. While allow_system_table_mods is not a very elegant solution to this,
we can't leave it as it is, so for now, revert it back to the way it was
previously.

src/backend/bootstrap/bootparse.y

@@ -222,7 +222,8 @@ Boot_CreateStmt:
 												   RELKIND_RELATION,
 												   RELPERSISTENCE_PERMANENT,
 												   shared_relation,
-												   mapped_relation);
+												   mapped_relation,
+												   true);
 						elog(DEBUG4, "bootstrap relation created");
 					}
 					else