show/reply: Unify the code that extracts text parts

show/reply: Unify the code that extracts text parts

Previously, show and reply had separate implementations of decoding
and printing text parts.  Now both use show's implementation, which
was more complete.  Show's implementation has been extended with an
option to add reply quoting to the extracted part (this is implemented
as a named flag to avoid naked booleans, even though it's the only
flag it can take).