Previously, we would find DNS servers we wish to intercept traffic on
by reading /etc/resolv.conf. On systems using systemd-resolved,
/etc/resolv.conf points to localhost and then systemd-resolved
actually uses the DNS servers listed in
/run/systemd/resolve/resolv.conf. Many programs will route the DNS
traffic through localhost as /etc/resolv.conf indicates and sshuttle
would capture it. However, systemd-resolved also provides other
interfaces for programs to resolve hostnames besides the localhost
server in /etc/resolv.conf.

This patch adds systemd-resolved's servers into the list of DNS
servers when --dns is used.

Note that sshuttle will continue to fail to intercept any traffic sent
to port 853 for DNS over TLS (which systemd-resolved also supports).

For more info, see:
sshuttle issue #535
https://www.freedesktop.org/software/systemd/man/systemd-resolved.service.html
https://github.com/systemd/systemd/issues/6076