Deprecating the FRAG Field in SOCKS5

This document updates RFC 1928 by formally deprecating the FRAG (Fragment) field in the SOCKS5 UDP ASSOCIATE request header. It mandates that the FRAG field MUST be set to X'00' by clients and that proxies MUST drop any SOCKS5 UDP packets containing a non-zero FRAG value. This change aligns the SOCKS5 protocol with modern Internet engineering practices regarding UDP fragmentation (BCP 227), simplifies proxy implementations, and eliminates a vector for resource exhaustion attacks.