Various 8259 device model improvements:

- implement 8259 "polled" mode.
- set 'atpic->sfn' if bit 4 in ICW4 is set during master initialization.
- report error if guest tries to enable the "special mask" mode.

Differential Revision:	https://reviews.freebsd.org/D1328
Reviewed by:		tychon
Reported by:		grehan
Tested by:		grehan
MFC after:		1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275952 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f