zsh: Remove obsolete environment variables.

My old audioserver (nasd) doesn't exist anymore since years, so let's
drop that stuff for now, especially because nowadays I'd probably use
pulseaudio or jack.

The SCUMMVM_PORT is only used for MIDI output, but if I should get back
to using ScummVM again, I'd probably patch the port into it directly.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
zsh: Set window title to command if using xterm.

Having a shitton of xterms open can get quite confusing at some point,
so let's make this easier by providing a sane title.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>