config: return the new InputOption from add_option.

Change add_option to return the new InputOption on success, or NULL
failure. This way we can at least check for errors in callers.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Daniel Stone <daniel@fooishbar.org>

config/config-backends.h

@@ -30,7 +30,7 @@
 
 void remove_devices(const char *backend, const char *config_info);
 BOOL device_is_duplicate(const char *config_info);
-void add_option(InputOption **options, const char *key, const char *value);
+InputOption* add_option(InputOption **options, const char *key, const char *value);
 
 #ifdef CONFIG_UDEV
 int config_udev_init(void);