Allow destroying a cgroup without freezer subsystem.

Currently, cgroups::destroy only works properly (killing procseses + remove cgroups) if freezer subsystem is available in the hierarchy. This is rarely true. Imagine we want to remove a cgroup in memory subsystem, and if there is a process still running, the cgroups::destory will fail. The dependency on the freezer subsystem during cgroups::destroy is not strictly necessary. We can probably just loop through all the tasks in a cgroup and send SIGKILL to them until all processes are gone (by checking the cgroups procs file).

This problem caused issues when running 'sudo make check' during 0.23 release testing, where BenH provided already a better error message with b1a23d6a52c31b8c5c840ab01902dbe00cb1feef / https://reviews.apache.org/r/36604.