MUX AEh interface wrong and incomplete #194

As noted in SvarDOS/bugz#6 (comment)

I think MS-DOS v4.01 and v6.22 append will fail, because you're passing 0 in DI (just like FreeCOM and possibly 4DOS). In MS-DOS, ES:DI on 2F.AE01 passes a far pointer for a far-callable entrypoint for some parsing: https://anonymous:github@hg.pushbx.org/ecm/msdos4/file/c661c6316db2/src/CMD/COMMAND/tmisc1.nas#l173 I verified that this is both provided by the caller (shell) and used by the callee (append) in both of these MS-DOS versions, and FreeCOM crashes when an MS-DOS append command is run.

Also, as noted in LoopZ/TheList#123 at least for function AE01h, the length that FreeCOM passes in CX is wrong. It's supposed to be the command name length (equal to byte [ds:si]), and not negative. CH is always zero. AE00h isn't as clear in the lDOS source texts so CH = 0FFh may be correct there.