notmuch show: Don't do text conversions for non-text parts

This fixes the recently-added test case (in test/multipart)
demonstrating corruption of binary parts that happen to contain CRLF
pairs.

We restore the original code from show_one_part_content to
format_part_content_raw. Then, for good measure, we rename
show_part_content to the more descriptive show_text_part_content and
add an internal error if it is ever called with a non-text part.