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/cputc.s

@@ -96,8 +96,8 @@ BAD_CHAR:
         jmp     plot
 
 ;-----------------------------------------------------------------------------
-; Initialize the conio subsystem. Code goes into the INIT segment, which may
-; be reused after startup.
+; Initialize the conio subsystem. "INIT" segment is nothing special on the
+; Creativision, it is part of the "ROM" memory.
 
 .segment        "INIT"
 
@@ -122,4 +122,4 @@ LL:     lda     boxchars,x
         bne     LL
 
         cli
-        jmp plot
+        jmp     plot