With the current naming it is common to have 'group' repeated 3 or
even 4 times in one line. This causes a number of readability
problems. Renaming it to cg reduces the amount of repetition.

The one place it is not renamed is in tst_test because it won't cause
repetition there. Meanwhile it is easier to search the internet for
'linux cgroup' than 'linux cg'.

Li Wang suggested renaming tst_cg to tst_cg_test because it is
consistent with tst_cg_drain. However I think tst_cg is used so often
that it is more important to have shorter lines.