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

[cockroach-teamcity]

cockroachdb/cockroach#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.