ftp/display: highlight the prompt message
ftp/makefile: add header file in the obj dependency
ftp/session: each ftp connection is represented by a session
ftp/session: client session in clients

For each client, there is only one session could be used now.
userspace code test: tcp
ftp/connection: add net_connection to handle network traffic

Also to move those network related information to net_connection.
ftp: add .gitignore
ftp/display: add interface to print message

Print a whole message with just a string.
Print one line message with formats.
ftp/command: add a help command