make gnutls priority configurable
When using kresd with DNS over TLS support the gnutls priority is hardcoded: https://github.com/CZ-NIC/knot-resolver/blob/c77bce8a0d2fe3e72542f87cf3914579d2bd8688/daemon/tls.c#L59
On some systems such as RHEL 7, this results in insecure settings. Redhat package maintainer have enabled SSL3 or RC4 support. "NORMAL" cipher suite list now supports insecure ciphers. A Redhat user can now either patch kresd or build gnutls himself.
It would be useful, if the priorities were a configuration setting to avoid this drawback.