backported fixes from 256boss:

 - moved early serial console from 1st to 2nd stage loader, to leave
   space for the partition table if needed.
 - saving boot_drive_number for later use.
 - fixed floppy motor turnoff after 2nd stage loader is done.
 - saving of carry flag before returning from int86.
 - added functions in intr.h/intr.c to manipulate the PIC IRQ masks.
 - restoring irq masks after returning from int86.
 - keyboard controller voodoo: do a single read after returning from
   int86 to clear any pending inputs that would otherwise clog the
   buffer and we'd never get an interrupt again from the keyboard.