src/streamssl: add possibility to store related handshake objects

The lifetime of SSL handshake objects should be tied to the stream the
handshake was done on instead of the proxy instance itself. This way the
handshake object(s) can be freed as soon as the SSL stream gets closed.

The idea is that none of our OpenSSL callbacks can be called once the
stream is closed so that's a safe point to free the related handshake
structure.

This patch adds the required infrastructure to store user data in an SSL
stream (together with its destroy method) so that the stream can be made
responsible of managing the lifetime of related handshake objects.
configure: Fixed compilation problesm with AC_C_CONST and -Werror.

AC_C_CONST autoconf macro cause compilation problems when -Werror option of
GCC, so -Werror should be used only to compile the package, not to configure
it.

http://lists.gnu.org/archive/html/bug-autoconf/2012-03/msg00070.html
src/connect: remove unused tos member variable
socket: move socket mark implementation from zorp-core to here
version: bump to 3.9.4.1