Major overhaul of mod_negotiation.c, part 2.

 - properly handle "identity" within Accept-Encoding.
 - allow encoded variants in RVSA negotiation and let them appear in
   the Alternates field using the non-standard "encoding" tag-list.
 - fixed both negotiation algorithms so that an explicitly accepted
   encoding is preferred over no encoding if "identity" is not
   included within Accept-Encoding.
 - added ap_array_pstrcat() to alloc.c for efficient concatenation
   of large substring sequences.  Bumped MMN.
 - replaced O(n^2) memory hogs in mod_negotiation with ap_array_pstrcat.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@82582 13f79535-47bb-0310-9956-ffa450edef68