Move linerva's Athena build chroot to schroot

[tabbott]

We should replace the Athena build chroot access mechanism on Linerva (currently using athmode, a custom C program written by tabbott) with schroot. This probably involves:

• Testing the athena chroot installer available in /mit/debathena/packages/config/athena-chroot, and getting an Athena chroot setup on mega-man (or a sipb-xen VM).
• configuring schroot to make sure /mit, /afs, and /tmp are mounted in the chroot and that they forward the KRB5CCNAME and KRBTKFILE environment variables.
• writing scripts to replace the sendmail, lastlog, and athmode programs by calling the appropriate program inside the chroot (all three should be very simple wrappers around schroot).
• It may be useful to backport schroot from lenny; this for example makes the mountpoint task easier.

[tabbott]

We should modify the installer script to remove setuid permissions after doing the Athena installation (or maybe a separate script).