CommandHandler for the MKD 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
- If the parent directory of the specified pathname does not exist, then reply with 550
- If the pathname parameter specifies an existing file or directory, or if the create directory fails, then reply with 550
- If the current user does not have write and execute access to the parent directory, then reply with 550
- Otherwise, reply with 257
The supplied pathname may be absolute or relative to the current directory.