Fixed a bug that caused 'make dist' unable to patch leveldb.

Review: https://reviews.apache.org/r/24061
Fixed the master to accept a file:// based zk flag.

Review: https://reviews.apache.org/r/23997
Converted bool returns to Try<Nothing> in stout/os.hpp.

Review: https://reviews.apache.org/r/20493
Mesos converted to use Try<Nothing> returns from stout/os.hpp.

Review: https://reviews.apache.org/r/20494
Cleaned up TODO comments for ZK file:// URL support.