Finally add precise/12.04 as a build-deb target

Ubuntu Precise has a number of warts that made it non-trivial to add initially, but I've managed to work through some of them and come up with a working build.  Two important parts to note are that it has neither the `btrfs` nor the `devicemapper` graphdriver backends since `btrfs-tools` and `libdevmapper-dev` in the precise repositories are too ancient for them to even compile.

Signed-off-by: Andrew "Tianon" Page <admwiggin@gmail.com>