Plugin unixsock: reenable getthreshold cmd.

The cmd getthreshold was disabled in the pass to version 5 when threshold become a plugin,  with the commit 59278c8774567122dbc0ea1a217d007782fa868e it's possible to enable again.

Also moved threshold_search and threshold_get to utils_threshold.c, may be there is more code duplicated in thredhold.c and write_riemann_threshold.c that can be moved to avoid duplicate code.

I'm think that this command it's very interesting to know that threshold applies if you have a complicated threshold setup.