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