fix segfault if there is error_page 401, proxy_intercept_errors is on
and backend does not return "WWW-Authenticate" header

ignore ngx_atomic_fetch_add() result
this fixes building at least by gcc 4.2.1 on Mac OS X 10.6

cache related fixes:

*) fix building --without-http-cache, broken in r2945
*) fix ./configure error message

various proxy/fastcgi cache features and fixes:

*) report about proxy/fastcgi_store and proxy/fastcgi_cache incompatibility
*) delete useless r->cache->uses
*) proxy_cache_use_stale/fastcgi_cache_use_stale  updating
*) inherit proxy_set_header, proxy_hide_header, and fastcgi_hide_header
   only if cache settings are similar
*) add response file uniq while loading cold cache on demand

test GIF87a

fix XSLT filter in SSI subrequests

fix a pool growing and CPU usage for multi-requests keepalive connections:

*) keepalive_requests
*) try to reuse last 4 free large allocation links
*) do not test a pool block space if we can not allocated from
   the block 4 times