Allow trailing commas in bare lists

This applies to macro arguments, DB, DW, DL, DS,
PRINT, PRINTLN, EXPORT, PURGE, and OPT.

It also removes support for empty entries in DB/DW/DL.
(Deprecating it would require keeping parser support,
which is ambiguous with trailing commas.)

Fixes #753

src/asm/rgbasm.5

@@ -1282,7 +1282,7 @@ You can also use
 .Ic DB , DW
 and
 .Ic DL
-without arguments, or leaving empty elements at any point in the list.
+without arguments.
 This works exactly like
 .Ic DS 1 , DS 2
 and