This change allows a callback to be used to override the generation of

SSL/TLS session IDs in a server. According to RFC2246, the session ID is an
arbitrary value chosen by the server. It can be useful to have some control
over this "arbitrary value" so as to choose it in ways that can aid in
things like external session caching and balancing (eg. clustering). The
default session ID generation is to fill the ID with random data.

The callback used by default is built in to ssl_sess.c, but registering a
callback in an SSL_CTX or in a particular SSL overrides this. BTW: SSL
callbacks will override SSL_CTX callbacks, and a new SSL structure inherits
any callback set in its 'parent' SSL_CTX. The header comments describe how
this mechanism ticks, and source code comments describe (hopefully) why it
ticks the way it does.

Man pages are on the way ...

[NB: Lutz was also hacking away and helping me to figure out how best to do
this.]