ircd/listener: return a fatal TLS alert for early rejected TLS clients

ircd/listener: return a fatal TLS alert for early rejected TLS clients

This is in furtherance of commit 3fdf26aa19628d5e12a3 which added
functionality to reply with a TLS record layer alert for D-Lined TLS
clients. It turns out that there are other plaintext error messages
in this same function that should receive the same treatment.

Also move another error string to a variable and use a compile-time
optimised-out strlen for it too, to use the same approach as an
existing error string.

Finally, use a different alert (internal_error) for the case where
IRCd is simply unable to accept more connections.