CommandHandler for the APPE command. Handler logic:
 
 - If the user has not logged in, then reply with 530 and terminate
 
 - If the required pathname parameter is missing, then reply with 501 and terminate
 
 - If the pathname parameter does not specify a valid filename, then reply with 553 and terminate
 
 - If the current user does not have write access to the named file, if it already exists, or else to its
 parent directory, then reply with 553 and terminate
 
 - If the current user does not have execute access to the parent directory, then reply with 553 and terminate
 
 - Send an initial reply of 150
 
 - Read all available bytes from the data connection and append to the named file in the server file system
 
 - If file write/store fails, then reply with 553 and terminate
 
 - Send a final reply with 226