Thread check types

[ssalinas]

Although cgroup isolation is common, posix is the default. If we are not using cgroup isolation we will not find a cpu cgroup to parse and this will error every time. This PR does a few things:

• Make two types of thread checks, CGROUP and PSTREE
• Configurably fall back to PSTREE when CGROUP fails (defaults to false)

/fixes #741

[tpetr]

I like the idea of removing the cgroups requirement for thread limiting, but our original pstree implementation wasn't great. For example:

$ pstree 91309 -p
daemon-runner(91309)───su(91330)───daemon-runner(91338)─┬─python2.6(91408)
                                                        ├─tee(91339)
                                                        └─tee(91340)

The thread checker would conclude that process 91309 is using 3 threads, when it's really using 6. Are there any other UNIX facilities we can use to figure out the thread count? Do we know what pstree is using under the hood in order to generate this info?

[ssalinas]

Good, point. Let me look and see what else we could use.

[ssalinas]

Updated to have three possible thread checking types:

• cgroup
• ps
• /proc//status

You can now configure the primary (defaults to cgroup) and a fallback (optional). If the fallback is set, it will be used in the case that the primary type does not result in a valid value