curl: SOCKS5 no-auth accepted despite username/password-only authentication

Summary: curl/libcurl appears to allow unauthenticated SOCKS5 negotiation even when the caller explicitly configures username/password-only SOCKS5 authentication. With --socks5-basic and SOCKS5 credentials set, curl still advertises both SOCKS5 method 0x00 no authentication and 0x02...