[rancid] Adding a separate module for Cisco IOS XR devices

[Per Carlson]

Hi.

While RANCID officially supports IOS XR (IOX) since 2.3.2, this
support is merely a tweak of the IOS-code, and not a proper IOS XR
module.

This tweak makes things suboptimal. For example you must turn on
autoenable for all XR devices and all commands are run from "user
mode" when some
commands benefits being run from "admin mode".

I would therefor propose to create a separate module for IOS XR
devices. A patch for creating this support is attached.