curl_json plugin: support arrays

Allow access to data in JSON arrays.  For example

    { workers: [ { requests: 10 }, { requests: 10 } ] }

can all be accessed with

    workers/*/requests

or just one by using the index.

    workers/1/requests
curl_json plugin: document being able to access arrays
curl_json plugin: separate the cj_curl_perform in to the curl and yajl parts
curl_json plugin: support getting json from a Unix socket as well as a URL
curl_json plugin: use sstrerr for thread safety
curl_json plugin: style cleanups
curl_json plugin: free the socket name
curl_json plugin: document the new Sock option
curl_json plugin: add an example of array access to the documented uWSGI example
curl_json plugin: warn when config expects a map and a number is provided

Moves the warning for strings in the number callback which was being called anyway.

This also serves to fix parsing numbers represented as JSON strings which was inadvertantly broken when adding array support.
Merge remote-tracking branches 'github/pr/392' and 'github/pr/399' into jr/json
Mention libsigrok dependency
curl_json plugin: Random minor improvements.

I did these changes while wrapping my head around the new array code.

* Added a comment to explain the cj_cb_map_key() function.
* Renamed some variabled and the arguments of the cj_cb_map_key() function.
* Renamed the "ignore" argument to "update_key" (inversing its meaning).
collectd.conf(5): Markup improvements for the curl_json plugin.
Merge branch 'jr/json'