* change nginx plugin for collectd to submit "nginx_accepts" and
* "nginx_handled"
replace nginx_{accepts,handled} DSs

reusing an existing DS is preferred. GH#227.
rrdtool plugin: Minor performance improvement.

Replace the value_list_to_string() function with a version which is optimized
for data sets with a single data source. The original function is still used
for data sets with multiple data sources. This allows to reduce the number of
calls to snprintf() in this (very common) case, resulting in ~2% speedup.