Change the resolved.conf Cache option to a tri-state "no, no-negative, yes" values.

If a lookup returns SERVFAIL systemd-resolved will cache the result for 30s (See 201d995),
however, there are several use cases on which this condition is not acceptable (See #5552 comments)
and the only workaround would be to disable cache entirely or flush it , which isn't optimal.

This change adds the 'no-negative' option when set it avoids putting in cache
negative answers but still works the same heuristics for positive answers.

Signed-off-by: Jorge Niedbalski <jnr@metaklass.org>