win32: change config path priorities

Assume mpv.exe is located in $mpv_exe_dir, then config files were
preferably loaded from "$mpv_exe_dir/mpv". This was mostly traditional,
and inherited from MPlayer times.

Reverse the config path priority order, and prefer $CSIDL_APPDATA/mpv as
main config path. This also fixes behavior when writing watch_later
configs, and mpv is installed in a not-writable path.

It's possible that this will cause regressions for some users, if the
change in preference suddenly prefers stale config files (which may
happen to longer around in the appdata config dir) over the user's
proper config.

Also explicitly document the behavior.
manpage: ipc: comment about invalid UTF-8

Some rationale for the documented/suggested behavior:

It's not really clear what to do with invalid UTF-8, since JSON simply
can't transport this information. Maybe you could transfer such strings
as byte arrays, but that would be very verbose and inconvenient, and
would pose the problem that it's hard to distinguish between strings
encoded in this way and actual arrays.

There are many other ways how this could be handled. For example, you
could replace invalid sequences with '?'. Or you could do it like
Python, and use certain reserved unicode codepoints to "tunnel" through
invalid bytes.

Which of these works really depends on the application. And since this
can be done entirely on the byte level (invalid UTF-8 sequences can
appear only in strings in our case), it's best to leave this to the
receiver.