add option to enable/disable port 92 emulation
DMA emulation bugfix: respond to page register read/writes for DMA
channels 0 and 4. contrary to original use, later PCs allow cards like
the Sound Blaster to play audio over DMA channel 0. widen DMA I/O port
assignment to cover all 8 I/O ports. add code to print warning message
if port register I/O not handled.
DMA bugfix: treat read OR write to port 0xC as flip flop reset, to
follow convention noticed on most motherboards.
move DMA init up among PIT initialization, where it belongs
add option to disable 2nd DMA controller, with note that DOSBox-X no
longer auto-disables the 2nd DMA controller when machine=cga unlike
mainline DOSBox.
add option to enable either or both DMA controllers
remove unused save state vars
bugfix: when the PIC has an IRQ 2 signal without a slave IRQ 8-15
signal, log an error and clear IRQ 2 instead of calling E_Exit(). Added
reminder to self to test what actual Sound Blaster 16 PnP cards do when
assigned IRQ 2 vs IRQ 9. Remember that on the ISA bus, the 8-bit
connector had an IRQ 2 (reserved) pin (PC/XT) that was reassigned to IRQ
9 on AT and later systems, and thus the ISA but doesn't really have IRQ
2 (but it's faked as IRQ 2 by DOS or the BIOS).

prior to this fix, running my Sound Blaster test program with
sbtype=sb16vibra and using the program to reassign the card to IRQ 2
caused DOSBox to E_Exit() with a complaint about IRQ 2 with no slave
IRQ, which imho is not an appropriate way to handle the error.
bugfix: PIC emulation updated to map IRQ 2 to IRQ 9. added notes to self
about future option to disable the slave PIC and emulate the master PIC
like a PC/XT system.
add comment to sblaster.cpp about allowing ISA PnP to set the IRQ == 2
and the change to PIC_ActivateIRQ to make the setting work regardless.