Add installation user to sudoers.

[andy pugh <bodgesoc@gmail.com>]

I have made a live image, with installer, and it seems to work as I
want, apart from one wrinkle.

I want the new user to be part of the sudoers group by default. and
for sudo to be available.
It seems that if you give a root password during the installation then
sudo is disabled.

Is there any way to change this behaviour? Many elements of our
software system and configuration require sudo to be used. I would
rather not leave the possibility of snookering oneself so early in the
process.

-- 
atp
"A motorcycle is a bicycle with a pandemonium attachment and is
designed for the especial use of mechanical geniuses, daredevils and
lunatics."
— George Fitch, Atlanta Constitution Newspaper, 1912