Return meaningful error for meaningless Dockerfiles
Make blank -H option default to the same as no -H was sent
Extract cgroups utilities to own submodule.

Use mountinfo rather than for cgroups parsing.
Make helper method private and change name.
Makes method naming more explicit rather than GetThisCgroup.
Use upstream term subsystem rather than cgroupType.
Merge pull request #3297 from jaseg/master

Return meaningful error for meaningless Dockerfiles
Merge pull request #3310 from pnasrat/cgroups-subpackage

Extract cgroups utilities to own submodule.
Merge pull request #3377 from cpuguy83/3333-fix_-H_default

Make blank -H option default to the same as no -H was sent