TelnetClient.php: Catch the correct Exception type in login()
TelnetClient.php: Allow null $login_prompt and/or $password_prompt
testTelnet.php: Fix --password-prompt
testTelnet.php: Set empty values as null for login and password prompt
testTelnet.php: Try harder to clean the output

TODO: Interpret and discard terminal characters
Add WIP procurveTest.php