Creativison changes.

This change includes some cleanups, removal of mainargs.s (game console
programs never have arguments), and a workaround for a problem I'm seeing.

The problem is that sometimes (in fact, more often than not) the clrscr()
call in testcode/lib/joy-test.c writes some garbage chars on the screen (most
often a "P"). Could be my hardware (I haven't seen it on MAME), but to
me the root cause is still unknown.

libsrc/creativision/clrscr.s

@@ -18,7 +18,7 @@ _clrscr:
         lda     #$50    ; VRAM offset high ($10 OR $40)
         sta     VDP_CONTROL_W
 
-        lda     #$C0    ; Space from ROM setup
+        lda     #$40    ; Space char from ROM setup
 
         ldx     #0
         ldy     #3
@@ -34,8 +34,8 @@ L1:     sta     VDP_DATA_W
         lda     #0
         sta     CURSOR_X
         sta     CURSOR_Y
-        sta     <SCREEN_PTR
+        sta     SCREEN_PTR
         lda     #$10
-        sta      >SCREEN_PTR
+        sta     SCREEN_PTR+1
 
         rts