Fix warnings from -Wimplicit-fallthrough

* plugin-runner.c (main/parse_opt): Add __builtin_unreachable() after
                                    calls to argp_state_help().
* plugins.d/askpass-fifo.c (error_plus): Add __builtin_unreachable()
                                         after calls to error().
* plugins.d/mandos-client.c (main/parse_opt) Add
                                             __builtin_unreachable()
                                             after calls to
                                             argp_state_help().
* plugins.d/password-prompt.c: - '' -