openvpn: cleanup and fix netifd proto script logic

openvpn: cleanup and fix netifd proto script logic

- Default to '--ifconfig-noexec' and '--route-noexec' to allow hotplug
  script to handle IP and routing configuration.
- Only append '--auth-user-pass-verify' if the option is explicitly
  configured to avoid unexpected authentication requirements.
- Fix missing retrieval of 'auth_user_pass', 'askpass', and 'tls_verify'.
- Remove redundant '--config' parameter (managed by option_builder).
- Simplify 'script_security' assignment logic.

Signed-off-by: Chen Minqiang <redacted>