TLS: make GnuTLS priorities compatible with CentOS 7 and Debian 9

gnutls-3.3.26-9.el7.x86_64 and libgnutls30-3.5.8-5+deb9u3 do not support
@SYSTEM keyword and CentOS 7 has problem with -VERS-DTLS-ALL.

We do not configure DTLS sockets so it should be harmless to delete
the DTLS keyword.

@SYSTEM is replaced by NORMAL, oh well.

fixup! TLS client: enforce minimal TLS version and no compression