yaml-cpp uses auto_ptr in a small number of places. This is problematic,
as libc++ 5.0.0 (used on HEAD) has auto_ptr disabled when building for
C++17. This causes the build for cloudabi-utils to fail on HEAD.
Instead of removing all of the use of auto_ptr from yaml-cpp, this
commit only changes public header files to make use of unique_ptr.
As I also ran into this issue for CloudABI, here's a bug report I filed
upstream a couple of weeks ago: