This allows other clients to insert headers in extra lines of text between
the Subject and Body fields of the message, as discussed on the 24x7 mailing
list. The PyBitmessage client was never able to meaningfully display
multi-line subjects, so this does not break anything. The extra lines are
thrown away and never stored anywhere, so this also protects against
watermarking attacks.
As per http://docs.python.org/2/howto/sockets.html#using-a-socket it's
possible that a socket recv() call returns 0 bytes if the remote closes
the connection. In that case, recv() does not obey settimeout(): it
just doesn't block and returns zero bytes immediately, which in this
case results in an infinite loop if the transmission was incomplete.
