Added utility functions in stout to return MAC addresses.

Review: https://reviews.apache.org/r/19556
Added utility functions to get IPv4 address of a given link device.

Review: https://reviews.apache.org/r/19602
Fixed MESOS-1055: explicit single-argument constructors.

Put "explicit" in front of all single-argument constructors in C++
Mesos source code. (Omitted lib process, filed a separate issue for
it.)

Review: https://reviews.apache.org/r/18823