Canonicalize our root path before we try using it, because we make assumptions about it not containing symlinks

Fixes #3242
Fix OSX build for sysinit
Fix OSX compilation for aufs
Add -a to our BUILDFLAGS directly, which fixes some fun test compilation issues

Also, now that we use "-a", we no longer get any benefit from "go test -i", and it actually causes problems sometimes, so let's nuke it.
Merge pull request #3266 from tianon/fix-integration-test-building

Add -a to our BUILDFLAGS directly, which fixes some fun test compilation issues
Merge pull request #3264 from creack/fix_osx_compilation

Fix osx compilation
Merge pull request #3263 from tianon/abspath-root

Canonicalize our root path before we try using it
Handle external mounts outside of lxc
Fix unmount issues