logind: Return proper error value when tty or seat doesn't match
logind: Return proper error value when tty or seat doesn't match

If the seat or tty doesn't match we return with r == 0, which looks like
success to weston_launcher_connect(), which then fails to fall back
to the legacy path.

https://bugs.freedesktop.org/show_bug.cgi?id=70876

Gnurou/weston,jonnylamb/weston,sir-murray/weston,eyolfson/weston,jonnylamb/weston,Gnurou/weston,Fantu/compositor-spice,mchalupa/weston,eyolfson/weston,kwm81/weston,udoprog/weston,Fantu/compositor-spice,krezovic/weston,kwm81/weston,xorgy/weston,Fantu/compositor-spice,giucam/weston,xorgy/weston,Tarnyko/weston-xdg_surface_present,mchalupa/weston,krezovic/weston,sir-murray/weston,Tarnyko/weston-xdg_surface_present,udoprog/weston,giucam/weston