[SV 46581] Pre-define .LOADED to avoid warnings.

* main.c (main): Pre-define .LOADED as a default-level variable.
* load.c (load_file): Set the value rather than append it.  Avoid
adding an extra initial whitespace.
* tests/scripts/features/load: Run with --warn-undefined-variables.

tests/scripts/features/load

@@ -56,7 +56,7 @@ load testload.so
 POST := $(.LOADED)
 all: ; @echo pre=$(PRE) post=$(POST) $(TESTLOAD)
 !,
-              '', "pre= post=testload.so implicit\n");
+              '--warn-undefined-variables', "pre= post=testload.so implicit\n");
 
 # TEST 2
 # Load using an explicit function