Make public ParseMountinfo

[BooleanCat]

Whilst using libcontainer/mount.GetMounts() I had need of the underlying logic of parsing /proc/self/mountinfo for testing purposes. This PR makes that logic public, allowing for parsing of a Reader whose contents look like /proc/self/mountinfo.

[BooleanCat]

@crosbymichael @cyphar Looks like there's an issue with runc's billing information for the pullapprove service.

[crosbymichael]

@caniszczyk for the pullapprove issues with billing

[AkihiroSuda]

LGTM

[kolyshkin]

@BooleanCat you can use "github.com/docker/docker/pkg/mount".GetMounts() (see https://github.com/moby/moby/blob/master/pkg/mount/mount.go)

I'm against making this one public, we might change it in the future and would have to maintain backward compatibility

[AkihiroSuda]

@kolyshkin Not huge fan of introducing dependency on moby/moby in this repo. Maybe the package should be split out to a separate repo?

[vrothberg]

How about moving the package from Moby here? I believe that (nearly) all projects needing it already have runc a code dependency. It would also avoid a circular dependencies between Moby and runc (on a project level).

[AkihiroSuda]

Closing per #2256 .
Moving moby/sys into opencontainers can be discussed in a separate issue.

[kolyshkin]

@BooleanCat let me update my recommendation: please use mountinfo package from https://github.com/moby/sys; for example, see #2256