weston-launch: Don't exit on SIGTERM
weston-launch: Don't exit on SIGTERM

Instead, forward signal to weston and wait for weston to clean up nicely.
Weston relies on weston-launch being around to shut down correctly,
so don't exit until we get the SIGCHLD from weston.  This make
killall weston-launch work properly.

https://bugs.freedesktop.org/show_bug.cgi?id=62910

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