Fix freeze in config's interrupt trap with some shells

With bash and zsh, the trap on the 5 second read does respond, but
doesn't break out of the read.  What's worse is that it takes away the
5 second timer, and therefore has the read hang indefinitely and
(almost) unbreakable.

Having the trap do 'exit 0' after reseting the tty params has it break
out of read and continue with the configuration.

Other shells do not appear to have the issue described here, but
neither does the extra 'exit 0' appear to harm them.

Reviewed-by: Andy Polyakov <appro@openssl.org>