SHELL: Introduce shellrancid, for backing up servers

Taken from:
http://www.shrubbery.net/pipermail/rancid-discuss/2010-October/005315.html

renamed to shell* because I want to backup BSD and Solaris too, but my
brain was hurting typing linux when I meant BSD ;)
AUTOTOOLS: Add in build instructions for shelllogin / shellrancid
SHELL: Life is better with instructions!