dnsdist: syslog should be enabled by default

dnsdist: syslog should be enabled by default

Regression introduced with the logging refactoring of dnsdist.

In rel/dnsdist-1.8.x: https://github.com/PowerDNS/pdns/blob/4d5bb67a2a75f9d88894e7dfc42bbbebfda297b0/pdns/dnsdist.cc#L103 :

bool g_syslog{true};

In master and 1.9.x the newly intrdoduced LoggingConfiguration::s_syslog
is inited to false.

This does not matter using the default systemd unit file as it disables syslog:

ExecStart=@bindir@/dnsdist --supervised --disable-syslog

but it does matter for non-systemd cases.