avoid the word "use" since it's better to show first use-certificate instead of this niche function.

even if textlen is zero, it still complains in syslog that the string passed is NULL (which kind of makes sense actually.)

This adds use-certificate, a user function to start using a certificate or switch to a different one. It asks whether to persist the certificate, if not it will only be used for the current session. use-certificate is implicitly called when the server replies with a 6x status code.