This adds command-line support to s_server for controlling the generation

of session IDs. Namely, passing "-id_prefix <text>" will set a
generate_session_id() callback that generates session IDs as random data
with <text> block-copied over the top of the start of the ID. This can be
viewed by watching the session ID s_client's output when it connects.

This is mostly useful for testing any SSL/TLS code (eg. proxies) that wish
to deal with multiple servers, when each of which might be generating a
unique range of session IDs (eg. with a certain prefix).