Attempt to fill the input buffer from the input source, returning a true flag if successful.

When the input source is the user input device, attempt to receive input into the terminal input buffer. If successful, make the result the input buffer, set >IN to zero, and return true. Receipt of a line containing no characters is considered successful. If there is no input available from the current input source, return false.

When the input source is a string from EVALUATE, return false and perform no other action.