Fix newline bug by escaping LF and CR in message

in-toto has new error messages that contain newline characters.
When relaying these messages to apt the text-based protocol breaks.
We fix this by escaping any newline character in the message.

NOTE: The apt method interface spec references RFC822, which
doesn't allow LF or CR in the message body, except if followed by a
LWSP-char (i.e. SPACE or HTAB, for "folding" of long lines). But
apt does not seem to support folding, and splits lines only at LF.
To be safe we escape LF and CR.  See 2.1 Overview in
www.fifi.org/doc/libapt-pkg-doc/method.html/ch2.html See "3.1.1.
LONG HEADER FIELDS" and  "3.1.2. STRUCTURE OF HEADER FIELDS" in
www.ietf.org/rfc/rfc822.txt

intoto.py

@@ -423,6 +423,16 @@ def write_one(message_str, stream):
 
 
 def notify_apt(code, message_text, uri):
+  # Escape LF and CR characters in message bodies to not break the protocol
+  message_text = message_text.replace("\n", "\\n").replace("\r", "\\r")
+  # NOTE: The apt method interface spec references RFC822, which doesn't allow
+  # LF or CR in the message body, except if followed by a LWSP-char (i.e. SPACE
+  # or HTAB, for "folding" of long lines). But apt does not seem to support
+  # folding, and splits lines only at LF. To be safe we escape LF and CR.
+  # See 2.1 Overview in www.fifi.org/doc/libapt-pkg-doc/method.html/ch2.html
+  # See "3.1.1. LONG HEADER FIELDS" and  "3.1.2. STRUCTURE OF HEADER FIELDS" in
+  # www.ietf.org/rfc/rfc822.txt
+
   write_one(serialize_one({
       "code": code,
       "info": MESSAGE_TYPE[code]["info"],