curl*: remove unneeded option declarations

They are equivalent to CURLOPT_USERPWD used a couple of line above and
aren't supported by curl versions older than 7.19.1
curl*: use CURLOPT_USERNAME if libcurl is recent enough

CURLOPT_USERNAME and CURLOPT_PASSWORD were introduced in curl 7.19.1.
They supersed CURLOPT_USERPWD which couldn't handle colons in the
username or password.
misc additions to curl* plugins doc & examples
Merge pull request #695 from mfournier/curlopt_username-fixups

Allow curl digest feature to build against older libcurl versions