bin: Tidied up assia-related scripts.
mutt: Change cwd to ~/transfers, so attachments are saved there by default.
urxvtd: Use xdg-basedir compliant location for socket.
mutt: Move the header cache into ~/.cache, instead of a subdirectoy of it.

When cleaning ~/.cache, nothing recreates ~/.cache/mutt, resulting in mutt no longer using a header cache.
Moving the cache to this new location prevents this failure.