irc: implement support for DANE

Use gnutls-dane to check DANE TLSA records on SSL connections.

This is currently performing synchronous DNS lookups which needs
to be changed. This will require a fix for the following GnuTLS
bug: https://savannah.gnu.org/support/index.php?108550

It also needs to depend on the version of gnutls that fixes the
following bug otherwise it will not be appropriate for connecting
to IRC servers (which are more likely to have multiple records):
https://savannah.gnu.org/support/index.php?108549

There is a further GnuTLS bug that makes it impractical to use
DANE TLSA records on IRC networks:
https://savannah.gnu.org/support/index.php?108551