Revert sqlite3_enable_load_extension() to its original long-standing behavior.

Add SQLITE_DBCONFIG_ENABLE_LOAD_EXTENSION which will enable only the C-API and
leave the SQL function disabled.

FossilOrigin-Name: b2ae5bfa32e608625bd177907596df3dbc2212e1

src/test1.c

@@ -6988,6 +6988,7 @@ static int test_sqlite3_db_config(
     { "FKEY",            SQLITE_DBCONFIG_ENABLE_FKEY },
     { "TRIGGER",         SQLITE_DBCONFIG_ENABLE_TRIGGER },
     { "FTS3_TOKENIZER",  SQLITE_DBCONFIG_ENABLE_FTS3_TOKENIZER },
+    { "LOAD_EXTENSION",  SQLITE_DBCONFIG_ENABLE_LOAD_EXTENSION },
   };
   int i;
   int v;