Added generic process utilities to libstout.

Review: https://reviews.apache.org/r/11092
Updated the ProcessIsolator to use the new libstout process utilities.

Review: https://reviews.apache.org/r/11094
Added utilities for getting process group and session members.

Review: https://reviews.apache.org/r/11847
Added a zombie bit to os::Process, improved os::alive to handle
zombie processes.

Review: https://reviews.apache.org/r/11848
Implemented a killtree utility (inspired by the killtree.sh script
in mesos).

Review: https://reviews.apache.org/r/11849
Removed killtree.sh in favor of os::killtree from libstout.

Review: https://reviews.apache.org/r/11853
Fixed environment.cpp to correctly handle an empty test filter.

Review: https://reviews.apache.org/r/11888
Adjusted the sched.cpp log levels based on importance.

Review: https://reviews.apache.org/r/11886
Fixed command executor to issue SIGKILL over SIGTERM and ensured
only one killtree operation is performed.

Review: https://reviews.apache.org/r/11986
Fixed the Freezer to ignore proceses that go away while in the
FREEZING state.

Review: https://reviews.apache.org/r/12019
Refactored Reaper with an abstraction that hides the dispatch syntax.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/11552
Added (EXPECT|ASSERT)_NONE in gtest.hpp.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/11555
Changed reaper to use a Future as the notification mechanism.
The notification is now sent only if the pid is explicitly registered
via the monitor() call.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/11554
Changed reaper to use an Option for the process exit status.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/11556