Added basic login function to the client and modified how the server responds

*All the code for logging in from the client side was moved to a
single function (probarly later moved to separate file)

*Added command_error(...) function to process_command.c that handles
sending the client an failure message (or any other type), closes
the socket and exits with status 0

*Added basic response mechanism in server to client communication:
	OK  : Everything ok, proceed
	BAD : Something went wrong, try again or forfeit operation
	FAIL: Something happened that makes the server drop the connection
		(like repetetive failure to provide proper credentials)

*The loging in code in the client and server (in process_client_commands.c)
seems almost production ready after some tweaks