APM BIOS emulation: add support for 16-bit and 32-bit protected mode
entry points (and connecting to them). Add options to dosbox.conf to
selectively allow/disallow the real, 16-bit and 32-bit protected mode
entry points (for diagnostic purposes). Add commentary that the entry
point hack (making a callback out of our INT 15h handler!) is an ugly
hack and needs to be refined later. Add code to report to the OS whether
or not the protected mode entry points are present. This fix allows
Windows 98's APM BIOS driver to work. Apparently Windows 98 demands that
the APM BIOS offer a 32-bit entry point/connection, and if the 32-bit
entry point is not available, then Windows 98 reports the APM BIOS as
"not working". Contrast that with Windows 95 and earlier that will
happily use either 16-bit or 32-bit entry points.