This commit introduces the cgroup-driven watchdog. It can be
initialized by calling watchdog.CgroupDriven().
This watchdog infers the limit from the process' cgroup, which
is either derived from /proc/self/cgroup, or from the root
cgroup if the PID == 1 (running in a container).
Tests have been added/refactored to accommodate running locally
and in a Docker container.
Certain test cases now must be isolated from one another, to
prevent side-effects from dirty go runtimes. A Makefile has been
introduced to run all tests.
Raúl Kripalani