Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Import libyaml as libbsdyml (private brand name)

LibYAML is a YAML 1.1 parser and emitter under MIT license which will
soon be used by the pkg boostrap (usr.bin/pkg) and bhyve

Reviewed by: roberto, antoine

Import libyaml 0.1.4 into vendor