When using only 1 second precision, curl doesn't create new cnonce values quickly enough for all uses.
For example, issuing the following command multiple times to a recent Tomcat causes authentication failures:
curl --digest -utest:test http://tomcat.test.com:8080/manager/list
This is because curl uses the same cnonce for several seconds, but doesn't increment the nonce counter. Tomcat correctly interprets this as a replay attack and rejects the request.
When microsecond-precision is available, this commit causes curl to change cnonce values much more frequently.