MINOR: htx: Add a function to append an HTX message to another one

the htx_append_msg() function can now be used to append an HTX message to
another one. All the message is copied or nothing. If an error occurs during the
copy, all changes are rolled back.

This patch is mandatory to fix a bug in http_reply_and_close() function. Be
careful to backport it first.
2 files changed