Refactored subprocess to support more IO redirect modes.

Review: https://reviews.apache.org/r/22688
Updated mesos to use the new Subprocess API.

Review: https://reviews.apache.org/r/22702
Allowed launching subprocesses with flags.

Review: https://reviews.apache.org/r/22749
Updated mesos to use the new Subprocess interface with flags.

Review: https://reviews.apache.org/r/22751
Added logging for ip resolution errors in libprocess.

Review: https://reviews.apache.org/r/22687
Added process delegate for accessing the scheduler endpoint.

Review: https://reviews.apache.org/r/22703/
Refactored os::user to return a Result.

Review: https://reviews.apache.org/r/22722/
Updated mesos to use refactored os::user call.

Review: https://reviews.apache.org/r/22722/
Updated reviewbot to only build and test once.
Added a SIGUSR1 handler to cleanly shut down the slave.

Review: https://reviews.apache.org/r/21379/
Added a SIGUSR1 shutdown test.

Review: https://reviews.apache.org/r/22737
Assert slave not master.

Test asserting on master that has to be slave.

Review: https://reviews.apache.org/r/22779
Fixed a compiler error due to ambiguous UNREACHABLE in Subprocess.
Added timeout to rescind unused offers
Added unit tests and fix segfault