Garbage collection in divert(4).

[glebius]

Summary

• Remove div_ctlinput().

This function does nothing since 97d8d152c28b. It was introduced
in 252f24a2cf40 with a sidenote "may not be needed".

• Don't run ip_ctloutput() for divert socket.

It was here since divert(4) was introduced, probably just came with a
protocol definition boilerplate. There is no useful socket option
that can be set or get for a divert socket.

[donner]

Thank you for taking care of this.