During line editing, let Ctrl-C discard the current input line and

During line editing, let Ctrl-C discard the current input line and
provide a fresh prompt instead of exiting the program.  This aligns
behaviour with bc(1), ftp(1), sftp(1), and all the shells.

OK naddy@

Both martijn@ and naddy@ point out that this program might profit
from signal handling during more of its code, but that would require
more code inspection, design work, and testing which i'm not planning
to do right now.  Consider standard behaviour during line editing
as a first step.