Avoid sending a trailing dot in SNI as a client

Avoid sending a trailing dot in SNI as a client

While an FQDN includes a trailing dot for the zero-length label of
the root, SNI explicitly does not contain it. Contrary to other TLS
implementations, our tlsext_sni_is_valid_hostname() rejects a trailing
dot. The result is that LibreSSL TLS servers encountering an SNI with
trailing dot abort the connection with an illegal_parameter alert.

This fixes an issue reported by danj in nc(1) and by sthen in ftp(1).
DNS cluebat from florian.

ok jsing