CommandHandler for the CWD command. Handler logic:
- If the user has not logged in, then reply with 530
- If the required pathname parameter is missing, then reply with 501 and terminate
- If the pathname parameter does not specify an existing directory, then reply with 550 and terminate
- If the current user does not have execute access to the directory, then reply with 550 and terminate
- Otherwise, reply with 250 and change the current directory stored in the session
The supplied pathname may be absolute or relative to the current directory.