set $ATHENA_USER everywhere

[geofft]

As described in #413, it's easier to assume $ATHENA_USER is always set instead of making sure everything supports ${ATHENA_USER:-$USER}. We almost always set $ATHENA_USER, but there are not-particularly-pathological circumstances under which it can fail to be set; we can either do this more aggressively in dotfiles or abuse PAM.

dr-wily:~ geofft$ ssh kid 'echo $ATHENA_USER'

dr-wily:~ geofft$ ssh kid 'echo $USER'
geofft