cmd/geth: exit the console cleanly when interrupted
cmd/geth: exit the console cleanly when interrupted

This fix applies mostly to unsupported terminals that do not trigger the
special interrupt handling in liner. Supported terminals were covered
because liner.Prompt returns an error if Ctrl-C is pressed.

maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum