app: Work around pkexec restriction when launching

Unless automatic child reaping is disabled in GSpawnFlags, glib's
g_spawn_* functions will reparent the spawned process to init by
double-forking. Unfortunately pkexec bails out in this case[0],
which means that it no longer works in .desktop files since the
PID tracking removal in commit 01c6392c.
Fix this by going back to manual child reaping.

[0] https://cgit.freedesktop.org/polkit/tree/src/programs/pkexec.c#n728

https://bugzilla.gnome.org/show_bug.cgi?id=763531