[MERGE] RFC-compliant object invalidation behaviour.

RFC-compliant object invalidation behaviour.
  
- Switch the default from not purging if the method is unknown to purging if
  the method is unknown.
- When purging URIs sourced from Location and Content-Location headers, make
  sure the URL is absolute before a) comparing it to see if hosts match and b)
  actually trying to find it in the store.

These changes are based on changes made to squid 2. In particular, the
urlAbsolute function was ported from squid 2. I would appreciate it if people
paid particular attention to urlAbsolute to make sure I'm not doing anything
that would cause problems in squid 3.