libplugin: plugin_exit helper which flushes stdout.

We weren't actually getting the last log out; this does that.

We have to fix test_bitcoin_failure which now notices the BROKEN
log message.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Fixed: libplugin: Fatal error messages from plugin_exit() now logged in lightningd.

plugins/libplugin.h

@@ -164,6 +164,9 @@ struct command_result *command_param_failed(void);
 /* Call this on fatal error. */
 void NORETURN plugin_err(struct plugin *p, const char *fmt, ...);
 
+/* Normal exit (makes sure to flush output!). */
+void NORETURN plugin_exit(struct plugin *p, int exitcode);
+
 /* This command is finished, here's a detailed error; @cmd cannot be
  * NULL, data can be NULL; otherwise it must be a JSON object. */
 struct command_result *WARN_UNUSED_RESULT