Require GnuTLS 3.6 and rework priority string generation

Require GnuTLS 3.6 and rework priority string generation...

...and drop support for unsafe renegotiation and protocol version fallback, both of which are already deprecated and haven't been needed for several years.

In particular, this allows us to stop hardcoding the NORMAL priority and append to the default priority instead, which makes no difference on most distros but is important for Fedora and RHEL, where we have to use the Fedora/RHEL-specific @SYSTEM@ priority instead of NORMAL. This means we can stop patching glib-networking downstream to change NORMAL to @SYSTEM@.

OpenSSL should like this too, since it never supported either of these properties, and since this allows removing the protocol version fallback test that it never passed.