[MERGE] Further cleanup of urlAbsolute and friends.

Ok, hopefully this meets with everybody's approval. Changes in this version:

- Split urlAbsolute into urlIsRelative and urlMakeAbsolute.
- Make urlIsRelative compliant with the RFC as to what defines a relative URL.
- Use a malloc'ed buffer instead of a stack-allocated array and xstrdup in
  urlMakeAbsolute.
- Rework purgeEntriesByHeader to be a little easier on the eyes.