libct/cg: make Set accept configs.Resources

A cgroup manager's Set method sets cgroup resources, but historically
it was accepting configs.Cgroups.

Refactor it to accept resources only. This is an improvement from the
API point of view, as the method can not change cgroup configuration
(such as path to the cgroup etc), it can only set (modify) its
resources/limits.

This also lays the foundation for complicated resource updates, as now
Set has two sets of resources -- the one that was previously specified
during cgroup manager creation (or the previous Set), and the one passed
in the argument, so it could deduce the difference between these. This
is a long term goal though.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/container_linux_test.go

@@ -42,7 +42,7 @@ func (m *mockCgroupManager) Apply(pid int) error {
 	return nil
 }
 
-func (m *mockCgroupManager) Set(container *configs.Config) error {
+func (m *mockCgroupManager) Set(_ *configs.Resources) error {
 	return nil
 }