textonly: add -q to quote the kept body

-q prefixes each line of the produced body with '> ' or a bare '>' for
empty lines. Quoting is done here, where the body is generated, so the
prefix bytes are counted into the reported length and the Content-Length
stays exact with no extra plumbing. This way the user can easily just
drop a level of indent in the whole message using vertical selection
when replying and have the text properly formatted to split the original
message from the added parts.