MINOR: listener/ssl: set the SSL xprt layer only once the whole config is known

MINOR: listener/ssl: set the SSL xprt layer only once the whole config is known

We used to preset XPRT_SSL on bind_conf->xprt when parsing the "ssl"
keyword, which required to be careful about what QUIC could have set
before, and which makes it impossible to consider the whole line to
set all options.

Now that we have the BC_O_USE_SSL option on the bind_conf, it becomes
easier to set XPRT_SSL only once the bind_conf's args are parsed.