athdir doesn't work in compatibility mode (ie: linuxbin)

[jdreed]

athdir -t bin -p /mit/joeuser -l

Should display a list of possible bin directories, ending with /mit/joeuser/linuxbin. Instead it ends with /mit/joeuser/%mbin.

This has been broken on Linux under Athena 9 for who knows how long. (See bugs[27293]). We should either fix it, or punt support for ancient bindirs. Either is fine with me, and I'd be slightly in favor of punting, because: a) then athdir should no longer care about $ATHENA_HOSTTYPE, and we can remove that from init/cshrc; b) Nothing in "linuxbin" directories is likely to work anyway.