Prior to this change a cgroup with a `:` character in it's path was not
parsed correctly (as occurs on some instances of systemd cgroups under
some versions of systemd, e.g. 225 with accounting).
This fixes that issue and adds a test.
Signed-off-by: Euan Kemp <euank@coreos.com>
GetMounts is very cpu-expensive. I'll change other funcs in this package
to reuse code from GetCgroupMounts later.
Signed-off-by: Alexander Morozov <lk4d4@docker.com>