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