Charybdis requires C99 already, so it's high time we start using
stdbool. I've converted a few pieces of code already.
A lot of the old code that uses YES/NO should probably be updated too
because that's fucking hideous.
It's possible for which_ssld to fail and return NULL, handle this in
start_ssld_connect and start_ssld_accept by returning NULL. The NULL
return value is already handled in all calls to start_ssld_accept,
so handle this for start_ssld_connect by reporting an error connecting.
Handle it in start_zlib_session by exiting the client.
