_updown code cleanup

Hi --

I was reading the _updown code and I noticed quite
a bit of parallelism between the prepare-client
code and the doroute code.

Observation:  There are 8 lines of code in
prepare-client that don't appear in doroute.
One could incorporate those lines in doroute,
triggered by an optional second argument to
doroute, such as
	doroute del OK-if-none
and then use that as the entire implementation
of prepare-client.

IMHO this would make the code more robust, more
maintainable, and more understandable to folks
who need to customize it.