collectd: Add the -T option.

collectd: Add the -T option.

The -T switch:

 - implies -f
 - does not start any read threads
 - invokes callback() foreach list_read and will exit 1 on failure, 0
   otherwise

Makes for a nicer dryrun than -f + ^C, also makes it possible to wrap
with a test harness such as Test.pm

Signed-off-by: Doug MacEachern <dougm@hyperic.com>
Signed-off-by: Florian Forster <octo@leeloo.lan.home.verplant.org>