Do not chmod +s chvt: instead, rely on sudo

The SUID flag may be lost on update, and relying on sudo is
slightly more secure.

We create a file in /etc/sudoers.d/ : these files are sourced by
default since sudo 1.7.2p1-1 (Ubuntu 10.04 lucid, Debian squeeze).

Fixes #723.
Merge pull request #727 from drinkcat/sudo-chvt

Do not chmod +s chvt: instead, rely on sudo