Cleanups for mig

[Justus Winter <4winter@informatik.uni-hamburg.de>]

Hi,

while waiting hours for package rebuilds, I looked at mig and started
by doing some cleanups. I hope noone shows up with a VAX and gcc < 2.0
and complains about me pulling the plug on the custom alloca
implementation ;)

I'm reasonably sure I didn't break anything, I rebuilt the Hurd with
this and it seems to work fine.

Justus