configure: Add gnutls-dane library
core: implement support for DANE

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

Performs a synchronous DNS lookup to get TLSA data between the
normal DNS resolve and the TCP connect.
irc: implement support for DANE

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

Performs a check of the cached TLSA data when verifying the
certificate.