Get TLS SNI server-switching working with GnuTLS.

Get TLS SNI server-switching working with GnuTLS.

Registering a cert/key in an x509 credentials *adds* them, and there's
no way to remove them, so we need a shiny new x509_cred each time the
key/cert change.

Since we avoid re-expanding unless tls_sni appears in tls_certificate,
we've mostly avoided the expense unless SNI is in use, and the extra
loading should be minimal, as everything should be in buffer/cache from
a few microseconds beforehand.

This code tested with GnuTLS and OpenSSL clients, without TLS
extensions, with servername, and verifying we do now get the correct
cert.