- extend ABI to create a code page for OS glue code
- initialize code page with trapframe glue
- extend traphandler to recognize code running in trapcode
  and simply reset the trap stack if the user was in the process
  of returning from a trap when the interrupt came (this avoids
  complicated nested trap handling/non-atomic POPA unwind
- generalize the syscall to force a trap synchronously, rename
  ioctly to more general DEVPASSIOCSYS
- Explicitly comment on the trap frame constructed in both pci_pass.c
  pci_pass_tramp.S