[Users] Revisiting the socket_dir $TMPDIR issue

[Michael Schwendt]

>   $ echo $XDG_RUNTIME_DIR
>   /run/user/1000
> 
> If $XDG_RUNTIME_DIR is not set, g_get_user_runtime_dir() falls back
> to $XDG_CACHE_HOME:
> 
>   $ unset XDG_RUNTIME_DIR
>   $ echo $XDG_CACHE_HOME
> 
>   $ xdg-user-dir RUNTIME
>   /home/ms21b

Unforunately, the opposite case, setting $XDG_RUNTIME_DIR to something
else, doesn't work with gnome-terminal's mailto handler either. The env
var also get's deleted. It's just the default that somehow works.