New code from lwip project

lwip-contrib repo:
28fe006 httpd: - separated content-types from other header fields to make it easier to add more content types (no manual indexing required); - added content-type for pdf, fixed content-type for javascript (no "x-");
4023aed httpd: - fix maximum write count for LWIP_HTTPD_DYNAMIC_FILE_READ==1 (was limited to 2*mss); - fix waiting for ACK from remote side after sending headers of async read files; - pass TCP_WRITE_FLAG_MORE when writing headers