XDG-fy mutt, add printing script for GNUGen, fix ZSH config

We may need to note that it requires an alias, not some fancy
environment variable :-(

GNUGen printing script simply launches lp on rainbowdash via SSH and
pipes the file into it.

ZSH config uses SSH_CONNECTION instead of SSH_TTY now. It seems more
reliable. Also the full host name is displayed now (doesn't usually
change anything, unless on a server like ayekat.ch).