curl_xml plugin: New plugin to fetch and parse XML files.

curl_xml plugin: New plugin to fetch and parse XML files.

Do find the curl_xml.c source file attached. The implementation is done as I
suggested in my previous mail.
The supported configuration is as follows:
<xpath "/path/to/node(s)">
   InstancePrefix "test" # optional
   Instance "path/to/textnode"  # optional if base xpath expression returns
single result i.e one matching node
   Values "path/to/textnode1"  "path/to/textnode2"
   Type "magic_level"
 </xpath>

I have done some level of testing to make sure plugin is working. If someone
is interested in testing/trying the plugin, do the following:
- apply the attached patch on the 4.9.0 branch
- copy the attached curl_xml.c to the collectd-4.9.0/src
- run autconf to generate new configure script containing changes to build
curl_xml plugin
- build and install collectd (make && make install)

Do let me know if anyone has problem building/running the plugin.

Any comments/suggestions are welcome.

Regards
Amit