Improve the SIOCSIFCAP handler a bit: - allow for ifp->if_ioctl being NULL, as the rest of ifioctl() does; - give the interface driver a chance to report a error to the caller; - don't forget to update ifp->if_lastchange upon successful modification   of interface operation parameters.
Improve the SIOCSIFCAP handler a bit:
- allow for ifp->if_ioctl being NULL, as the rest of ifioctl() does;
- give the interface driver a chance to report a error to the caller;
- don't forget to update ifp->if_lastchange upon successful modification
  of interface operation parameters.

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