introduce --with-distro=raspbian

introduce --with-distro=raspbian

Raspberry Pi kernel finally supports all the bits required by LXC [1]

This patch makes "./configure --with-distro=raspbian" to install lxcbr0
based config file and upstart jobs.
Also src/lxc/lxc.net now checks the existence of the lxc-dnsmasq user
(and fallbacks to dnsmasq)

RPI users still need to pass
"MIRROR=http://archive.raspbian.org/raspbian/" parameter to lxc-create
to pick the correct packages

MIRROR=http://archive.raspbian.org/raspbian/ lxc-create -t debian -n rpi

[Could be applied to stable-1.0 if you cherry-pick
7157a508ba3015b830877a5e4d6ca9debb3fd064]

[1] https://github.com/raspberrypi/linux/issues/176

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>