Fixed the broken build on OSX caused by cgroups::enabled().

Review: https://reviews.apache.org/r/21664
Minor fix to pre-commit hook.
Updated post-reviews to work with 'post-review' (RBTools < 0.6) and
'rbt' (RBtools >= 0.6).

Review: https://reviews.apache.org/r/21257
Add os::getgid() and os::getuid() using reentrant version of getpwnam.
Also, updated os::user() to use reentrant version of getpwuid.

Review: https://reviews.apache.org/r/20955
Add ExecEnv for use with execle to safely exec.

execle() should be used with envp to set a child's environment variables
in an async safe manner.

Review: https://reviews.apache.org/r/20956
Update Subprocess to use ExecEnv from stout.

Review: https://reviews.apache.org/r/20957
Make execute in MesosContainerizer async signal safe.

Test MesosContainerizer redirects task stdout and stderr.

Review: https://reviews.apache.org/r/20958
Cleaned up some style issues in MesosContainerizer
Correct include of stout/os/execenv.hpp in stout distribution
Added file permissions to open() call when O_CREAT is specified.

Review: https://reviews.apache.org/r/21778
Added mesos-log to the aggregate target.
Added mesos-tests.
Explicit conversion to Result<> from Unreachable to squash compiler error.

see summary

Without the explicit conversion some compilers find the conversion from Unreachable() to Result<T>() ambiguous.

Review: https://reviews.apache.org/r/21777
Added mesos-tests and reduced warnings down to 5.