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