Various 8259 device model improvements.

[neel]

Summary

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.

Reported and tested by Peter Grehan (grehan@).

Test Plan

Prior to this change, when installing Centos4.8/i386, bhyve exits due to an
unhandled i/o port access. This was tracked down to the vatpic, with polled
mode not being implemented.

After this change the installation works as expected.

[tychon]

So odd that SFN was implemented minus the ability to actually enable it. Seems like the priority to commit jumped the priority to complete ;-)

Thanks for finishing it! The changes look good to me.