Let user exit 'modem'

[hackerb9]

According to his man page, Fisher's original modem program exits when ^D (control-D) is sent. However, this revised version has to be killed with SIGHUP from a different terminal. (pkill -1 modem)

Please add the ability for the user to exit the program. I am not sure what would be best, but here's an idea:

• If in command mode (not connected), allow ^D (EOF), ^C (sigint), and ^\ (sigquit) to close the modem program.
• If in data mode (connected, e.g., after "ATA"), then let user return to command mode by typing pause +++ pause .

Please see the description of the Heatherington '302 in https://en.wikipedia.org/wiki/Time_Independent_Escape_Sequence for information on how the +++ escape sequence worked only when typed by a human, not when uploading files.