Seems like a valid use-case. Not sure if I like it calling back into the
config code. Care has to be taken for not letting the config path
resolving code dead-lock (which is why locking details in the msg.c code
The code for expanding the ~~ prefix used mp_find_config_file(), which
strictly looks for _existing_ files in any config path (e.g. not just
the user-local one, but also system-wide config). If no such file
exists, it simply returns NULL, which makes the code below just return
the literal, unexpanded path.
Change this so that it'll resolve the path to the user-local config
Requested in #3591.