- Add support for umount -f: it invalidates all in-flight and delayed requests

- Add support for umount -f: it invalidates all in-flight and delayed requests
  and marks the connection(s) as invalid, so that future operations fail.
- Make the NEWCONN ioctl clear the CONN_INVALID flag.
- Remove a handful of unused members from ptlrpc_request.
- More informative and uniform req-failure (timeout, recovery, umount -f)
  reporting.
- Update runfailure-net a little; more work needed here.
- Add --force / -f to lconf for "umount -f" umounting.