rename do_show_raw to do_show_single, and create params.raw for raw message output

We rename here in order to make do_show_single into a generic function
for handling output of just a single message, or which format=raw is a
special case.  The raw case is handled by setting a new parameter,
params.raw, which is used to tell do_show_single to output a single
message as a raw file.

This is mostly in preparation for much improved part handling to
follow imminently.