show: Convert text format to the new self-recursive style

show: Convert text format to the new self-recursive style

This is all code movement and a smidgen of glue.  This moves the
existing text formatter code into one self-recursive function, but
doesn't change any of the logic.  The next patch will actually take
advantage of what the new structure has to offer.

Note that this patch retains format_headers_message_part_text because
it is also used by the raw format.