This should fix the "warning: 'curl_strequal' redeclared without
dllimport attribute: previous dllimport ignored" message and subsequent
link error on Windows because of the missing CURL_EXTERN on the
prototype.
These two public functions have been mentioned as deprecated since a
very long time but since they are still part of the API and ABI we need
to keep them around.
... to make it less likely that we forget that the function actually
does case insentive compares. Also replaced several invokes of the
function with a plain strcmp when case sensitivity is not an issue (like
comparing with "-").
