FTP command injection via CRLF in control channel arguments

Summary

Affected versions of `suppaftp` wrote command arguments (user name, password, paths, `SITE` arguments and custom commands) to the FTP control channel without validation. An argument containing a carriage return (`\r`) or a line feed (`\n`) terminated the intended command line and let a second, attacker-chosen command be sent to the server within the same authenticated session.

An application that passes untrusted input as credentials, paths or command strings to methods such as `login`, `cwd`, `mkdir`, `rmdir`, `rm`, `rename`, `retr`, `put_file`, `site` or `custom_command` can therefore be made to execute arbitrary FTP commands with the application's privileges, for example deleting files or redirecting a data connection with an injected `PORT`.

All three clients are affected: sync, tokio and smol, with or without TLS.

The flaw was corrected in version 10.0.2 (commit [194bdd1](https://github.com/veeso/suppaftp/commit/194bdd1979b16c4848d1fad6897dfa524b688d88)): every command line is validated before it is written to the wire and rejected with `FtpError::ConnectionError` (`std::io::ErrorKind::InvalidInput`) if it contains CR or LF anywhere but in the trailing terminator. As a consequence, `custom_command` no longer accepts several commands joined by CRLF in a single call.

Users who cannot upgrade should reject or strip `\r` and `\n` from any untrusted string before passing it to the client.