Merge pull request #695 from mfournier/curlopt_username-fixups

Allow curl digest feature to build against older libcurl versions

diff --cc configure.ac
Simple merge

diff --cc src/collectd.conf.in
Simple merge

diff --cc src/collectd.conf.pod
Simple merge

diff --cc src/curl.c
Simple merge

diff --cc src/curl_json.c
Simple merge

diff --cc src/curl_xml.c
Simple merge