* Refactor out the common code from the byname and byoid paths into a new function.
* Handle a NULL oldlenp - this is the case for sysctl writes.

This (with the upcoming libuinet changes) allows for sysctl writing to occur.
(note: grr, should be using a generic C RPC thing already. This hand rolling crap
is annoying.)

* Disable the sysctlbyname path until I've refactored out the common code.
* Handle the no-respbuflen case - sysctl writes will call sysctl() with
  oldlenp=NULL.

This allows sysctl writes to occur.