curl_debug: prettify debug output

One line output used to be:
    curl rcvd header: 'Vary: Cookie
    '
This change makes it look like:
    curl rcvd header: 'Vary: Cookie^M^J'

which is more compact, and shows ^M (which is otherwise not visible).

Multi-line data will have only its _trailing_ ^Ms and ^Js
shown this way.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>