configs package can no longer be built on non-Linux OS, such as Darwin.
When running `GOOS=darwin go build` on the packge, we had the following
errors:
```
./configs/mount.go:34:16: undefined: unix.MountAttr
./configs/mount.go:47:22: undefined: unix.MS_BIND
```
Let's ensure that the linux specific bits are handled in mount_linux.go,
and introduce a _unsupported file, similar to how cgroups file is
handled within the package. This'll facilitate utilization of the pkg
for other projects that care about Darwin.
Signed-off-by: Eric Ernst <eric_ernst@apple.com>