Make kill_process_tree (in process.py) more robust on Mac OS X

[brycelelbach]

It should be possible to make the kill_process_tree code more effective for OS X using the same strategy that we use on linux (which basically amounts to recursively calling some invocation of ps - crude, but the best solution for finding a sub-tree of a process group). The only difference is that the OS X version of ps takes a slightly different syntax.

[brycelelbach]

Done

[hkaiser]

This is still failing: http://hermione.cct.lsu.edu/builders/hpx_clang33_x8664_boost154_debug/builds/420/steps/run_unit_tests/logs/stdio

[sithhell]

This is still not really working. See:
http://hermione.cct.lsu.edu/builders/hpx_clang33_x8664_boost154_release/builds/433/steps/run_unit_tests/logs/stdio

[brycelelbach]

I just retested the kill process code, it's fine. The real problem is the way the buildbot is configured: it runs both builders (release and debug) on the mac buildslave at the same time.