Function to actually send to PROCESS the STRING submitted by user.
Usually this is just `comint-simple-send', but if your mode needs to
massage the input string, put a different function here.
`comint-simple-send' just sends the string plus a newline.
(If `comint-input-sender-no-newline' is non-nil, it omits the newline.)
This is called from the user command `comint-send-input'.