Enable --sslcertck by default.

Enable --sslcertck by default.

There are no sslcertck (rcfile) and --nosslcertck (command line) options
that can be combined with [--]sslfingerprint if so desired.

The documentation is deliberately not updated everywhere, so that
recommendations to use --sslcertck stand, this is for the benefit of
users that read fetchmail v6.4.0 manuals to configure a fetchmail v6.3.X
implementation.