Standard shell ^C / blank-input / exit behavior

[NoRePercussions]

On ^C, standard shells and interpreters such as the python interpreter will stop the current command or discard the current input, but not exit the program. As engshell behaves like these shells, this is desirable behavior.

In addition, this PR ignores blank input along the same line of thought as above.

It changes the default way to exit the shell to the command exit (which also happens to work in the previous versions) in line with shells and the python interpreter (exit()). It displays a message about this when the shell is instantiated.