fusefs: improvements to interruptibility

fusefs: improvements to interruptibility

* If a process receives a fatal signal while blocked on a fuse operation,
  return ASAP without waiting for the operation to complete.  But still send
  the FUSE_INTERRUPT op to the daemon.
* Plug memory leaks from r346339

Interruptibility is now fully functional, but it could be better:
* Operations that haven't been sent to the server yet should be aborted
  without sending FUSE_INTERRUPT.
* It would be great if write operations could be made restartable.
  That would require delaying uiomove until the last possible moment, which
  would be sometime during fuse_device_read.
* It would be nice if we didn't have to guess which EAGAIN responses were
  for FUSE_INTERRUPT operations.

PR:		236530
Sponsored by:	The FreeBSD Foundation