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

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

Mark any distfiles with no checksum entries in the manifest "Skipped"
instead of "Passed".

Add generation of an installation manifest containing SHA256 checksums as
well as package descriptions and add code in the installer to check the
checksums.