Teach tramp to ssh then sudo

This involves configuring a tramp default proxy which interprets
/sudo:root@host[#port]:/path/to/file as meaning ssh to host[#port] as
yourself then sudo to root and access /path/to/file.  Kudos to Maxim
Filatov for this answer, which was not the chosen one on Stack
Overflow.[1]

[1] http://stackoverflow.com/questions/3465567/how-to-use-ssh-and-sudo-together-with-tramp-in-emacs.