merge r3153, r3154, r3288, r3382:

merge r3153, r3154, r3288, r3382:

request header processing fixes:

*) $host is always in low case:
   *) move low case convertation from ngx_http_find_virtual_server()
      to ngx_http_validate_host()
   *) add in ngx_http_validate_host() capability to copy host name in the pool
      allocated memory
*) fix segfault if there is single large_client_header_buffers
   and a request line fills it completely
*) default large_client_header_buffers' buffer size should be 8K as compatible
   with Apache's one