Tweaks to get it working on a Mac in 2013:

* Makefile: Use /bin/echo instead of echo: for some reason, the latter
  doesn't honor "-n"

* Makefile: Get rid of "zip" and "pdir" targets because I don't
  remember what they're for

* keys.ss: Look for "USER" if "USERNAME" isn't present in the
  environment

* gui.ss: Invoke main; apparently that's needed now