Kill off /bin/athena

[jdreed]

This is subtly different from #1263. For one thing, shells are in /bin/athena, and they need to be present to work. I'm happy to decide that we need to keep shipping /bin/athena/{bash,tcsh} symlinks forever, but it would be nice to update everything in Moira, if we can do so without seriously impacting older platforms. I think it's safe to update everyone's shell to be in /bin, and decide that if you regularly use platforms without a usable /bin/tcsh or /bin/bash, you're on your own. Similarly, if you depend on the Athena versions of tcsh and bash on older platforms, you're also on your own. We should also figure out what else in /bin/athena might still be hardcoded in various places.