Implement SNI support for the gnutls SSL plugin.

Note that gnutls is picky in regard to what it accepts as the server name - it
MUST be a domain name. IP addresses are not supported according to the
documentation.

Hence, filter out IP addresses and hope that whatever is not recognized as
such an address is actually a domain name. This will probably fail for more
exotic addresses (especially in IPv6 realm), but wiring up a full-blown parser
is too much effort and SSL plugins are not part of purple-3 anyway.

Fixes #17300