cli,sql: do not exit interactive shells with Ctrl+C

https://github.com/cockroachdb/cockroach/pull/76427 --- Release note (cli change): `cockroach sql` (and `demo`) now continue to accept user input when Ctrl+C is pressed at the interactive prompt and the current input line is empty. Previously, it would terminate the shell. To terminate the shell, the client-side command `%5Cq` is still supported. The user can also terminate the input altogether via EOF (Ctrl+D). The behavior in non-interactive use remains unchanged.