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

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

Allow pty(4) to be loaded as a kld.

Unfortunately, the wrappers that are present in pts(4) don't have the
mechanics to allow pty(4) to be unloaded safely, so I'm forcing this kld
to return EBUSY. This also means we have to enable some extra code in
pts(4) unconditionally.

Proposed by: rwatson