Simplify the CLI set_logging logic

Simplify the CLI set_logging logic

The CLI's set_logging logic seemed unnecessarily complicated to me.
This patch simplifies it, with an eye toward changing it to use RAII
objects in a subsequent patch.

I did not touch the corresponding MI code.  That code seems incorrect
(nothing ever uses raw_stdlog, and nothing ever sets
saved_raw_stdlog).  I didn't attempt to fix this, because I question
whether this is even useful for MI.