src/utils_cache.c: Update GETVAL output when missing state.

src/utils_cache.c: Update GETVAL output when missing state.

Hi

I think that I've found a bug when use unixsock plugin. The problem is
releate with missing state, when no value is received by daemon for a
while in the cache is marked as MISSING, but the last value is still
showing even when machine is not reporting in a GETVAL and LISTVAL
commands. Some utlities like collectd-nagios does not work correctly,
and report an OKAY value when host is not reporting from a long time.

I attach a patch with check the state value of an cache entry in
uc_get_names and in uc_get_rate_by_name. This patch works for me, but
it's not very tested yet, and I not very sure about if it's a good way
to check the problem. The patch is tested on 4.7.2 release version.

BTW a GETSTATE command will be an useful feature too :P

Regards,
  Andres

Signed-off-by: Florian Forster <octo@huhu.verplant.org>