Implement "special mask mode" in vatpic.

OpenBSD guests always enable "special mask mode" during boot. As a result of
r275952 this is flagged as an error and the guest cannot boot.

Reviewed by:	grehan
Differential Revision:	https://reviews.freebsd.org/D1384
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@276323 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f