This makes the new default $XDG_STATE_HOME/syncthing or
~/.local/state/syncthing, while still looking in legacy locations first
for existing installs.
Note that this does not *move* existing installs, and nor should we.
Existing paths will continue to be used as-is, but the user can move the
dir into the new place if they want to use it (as they could prior to
this change as well, for that matter).
### Documentation
Needs update to the config docs about our default locations.
