store Common Information Model name in ulogd key

This patch adds storage for CIM field name in ulogd key. This
will be used by JSON output to interoperate with logging
collector such as logstash or splunk.

Common Information Model is an open standard that defines how managed
elements in an IT environment are represented as a common set of objects
and relationships between them:
 http://www.dmtf.org/standards/cim

This seems to be mainly XML based but there is a JSON version of some
aspects of the model. One of the main documentation on CIM in JSON
format seems to be:
 http://docs.splunk.com/Documentation/PCI/2.0/DataSource/CommonInformationModelFieldReference

Using the correct CIM field name allow events coming from ulogd to be
correlated with events coming from other sources.
json: introduce new JSON output plugin

This patch introduces a new JSON output plugin. This
patch displays CIM field name instead of ulogd key valu
if this CIM field is available.

The module does not display binary address but uses the
string version of them. So a complete stack is for example:
 stack=log2:NFLOG,base1:BASE,ifi1:IFINDEX,ip2str1:IP2STR,mac2str1:HWHDR,json1:JSON

If boolean_label is set to 1, then the numeric_label put on packet
by the input plugin is coding the decision on packet. If 0, then
packet has been blocked and if non null it has been accepted.