Clean Ctrl-C exit from monitor.

[hjoliver]

Close #2762

[hjoliver]

(Not too taxing a review job :-)

[matthewrmshin]

Just wondering... What do we want the exit code to be? I guess cylc monitor is designed to exit via KeyboardInterrupt. In which case, the behaviour here is correct - simply exit (return 0). Do we want to print out a KeyboardInterrupt message to sys.stderr to inform the user that cylc monitor is killed normally? (Or maybe it does not matter.)

tail -f ... returns non-zero on Ctrl-C. And it prints Ctrl-C.
watch ... returns zero on Ctrl-C and does not print anything.

So there is no correct or incorrect behaviour, I suppose.

[hjoliver]

IMO exit 0 is fine in this case. And this does leave "^C" visible at exit (in my environment at least).

[sadielbartholomew]

Works 'cleanly' as described. N.b. '^C' is also visible after Ctrl-C in my environment, implying standard exit.