path: don't access global option struct

The path functions need to access the option that forces non-default
config directories. Just add it as a field to mpv_global - it seems
justified. The accessed options were always enforced as immutable after
init, so there's not much of a change.

common/global.h

@@ -8,6 +8,7 @@ struct mpv_global {
     struct mp_log *log;
     struct m_config_shadow *config;
     struct mp_client_api *client_api;
+    char *configdir;
 
     // Using this is deprecated and should be avoided (missing synchronization).
     // Use m_config_cache to access mpv_global.config instead.