system-source: parse @cim marked messages by json-parser automatically

This allows any local applications to submit JSON formatted messages
through the syslog() API, which will automatically be parsed. A ".cim."
prefix will be added to the fields extracted.

The same values can be reconstructed using a value-pairs expression,
something like:

  $(format-json --key .cim.* --shift 5)

Signed-off-by: Balazs Scheidler <bazsi@balabit.com>
Merge branch 'f/3.6/parse-cim-json-messages-in-system' into 3.6/master

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>