Redmine #6180: Introduce libyaml integration.

With readyaml() and parseyaml() functions.

Based on pull request #2028 in CFEngine core repository.
Modified by Kristian Amlie <kristian.amlie@cfengine.com>
Add extra acceptance test for parseyaml abuse.
Merge pull request #2054 from kacfengine/libyaml

Redmine #6180: Introduce libyaml integration.