It's time to refactor the handling of %disabled so that all
information of value is in the same place.  We have so far had a few
cascading disable rules in form of code, far away from %disabled.
Instead, bring that information to the array @disable_cascade, which
is a list of pairs of the form 'test => descendents'.  The test part
can be a string, and it's simply checked if that string is a key in
%disabled, or it can be a CODEref to do a more complex test.  If the
test comes true, then all descendents are disabled.  This check is
performed until there are no more things that need to be disabled.

Also, $default_depflags is constructed from the information in
%disabled instead of being a separate string.  While a string of its
own is visually appealing, it's much too easy to forget to update it
when something is changed in %disabled.
Reviewed-by: NViktor Dukhovni <viktor@openssl.org>