convert usbd_{intr,bulk}_transfer() in the USBMP world:
- add a new USBD_SYNCHRONOUS_SIG flag for transfers
- in usbd_transfer(), if USBD_SYNCHRONOUS_SIG is set use cv_wait_sig()
  (or tlseep(xfer, PZERO|PATCH, ...) for the unconverted controllers)
- add a usbd_sync_transfer_sig() front-end to usbd_transfer()
- greatly simplify both usbd_{intr,bulk}_transfer() to just
  usbd_sync_transfer_sig() and usbd_get_xfer_status().

this fixes lockdebug issues where usbd_{intr,bulk}_transfer() where it
taking the pipe lock, when usbd_transfer() would call functions that
expect the pipe lock not to be taken (and try to taken it.)

(mrg)