Added os::process and os::children that uses a Process list.

Review: https://reviews.apache.org/r/12428
Fixed os::sysctl::string to return results with null bytes.

Review: https://reviews.apache.org/r/12429
Added "argv" lookup for Process::command on OS X.

Review: https://reviews.apache.org/r/12430
Added os::pstree, os::pstrees, and os::ProcessTree.

Review: https://reviews.apache.org/r/12431
Added os::Fork for declaring and instantiating process trees.

Review: https://reviews.apache.org/r/12432
Updated proc::status to handle race with checking if a file exists and
opening the file.

Review: https://reviews.apache.org/r/12433
Fixed naming in a test.

Review: https://reviews.apache.org/r/12434
Added proc::cmdline to get better command lines in os::Process.

Review: https://reviews.apache.org/r/12435
Added implicit Option<T> to Result<T>.

Review: https://reviews.apache.org/r/12487
Updated os::killtree:

* Uses a static "view" of the processes (less system calls and less
  volatile semantics).

* Doesn't include processes from the parent group or session (unless
  they are in the tree).

* Returns a list of process trees instead of outputing the different
  processes encountered (the process trees can be stringified).

Review: https://reviews.apache.org/r/12436
Updated Mesos to use the new os::killtree.

Review: https://reviews.apache.org/r/12437
Refactored ZooKeeperState.java into AbstractState.java.

Review: https://reviews.apache.org/r/12441
Added LevelDBState.java.

Review: https://reviews.apache.org/r/12492