Import libyaml into base

hi,

I want to import libyaml into base as libbsdyml so that no ports will use it
like we do for expat.

I need it for the pkg bootstrap, so it it can parse pkg.conf.

I know that some of the bhyve people will also be glad to use it.

libyaml is MIT licensed, it is stable: no abi/api revolution in it for a while.

Does anyone have an objection?

regards,
Bapt