Change the default for building with net-snmp from `auto` to `no`

Change the default for building with net-snmp from `auto` to `no`

We have had a fair amount of issues with `net-snmp` adding unexpected
and unwanted values to both `CXXFLAGS` and `LDFLAGS`, and it was just
reported to also define `HAVE_LIBSSL` is in public header, which messes
up our own feature detection. Therefore it is likely better for everyone
to prevent net-snmp from being enabled without the user intending it.
For our own packages we explicitly enable net-snmp when supported,
and this commit also enables it in our CI for dnsdist (it was already
done for the recursor) so it should not have any impact.