Added linux /proc utilities in stout.

Review: https://reviews.apache.org/r/10809

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1477455 13f79535-47bb-0310-9956-ffa450edef68
Added proc_tests.cpp to the libprocess third party tests.

Review: https://reviews.apache.org/r/10810

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1477456 13f79535-47bb-0310-9956-ffa450edef68
Removed linux/proc.{hpp,cpp} in favor of stout/proc.hpp.

Review: https://reviews.apache.org/r/10811

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1477457 13f79535-47bb-0310-9956-ffa450edef68