xserver: ignore SIGUSR1 in the parent after server startup
xserver: ignore SIGUSR1 in the parent after server startup

SIGUSR1 is triggered on server regeneration too, not just on startup. And
any test that wants to handle SIGUSR1 for other stuff needs to set a signal
handler anyway.

Signed-off-by: Peter Hutterer <b9c50af7afa7642f6f854434cda9ffe54ece8b85@who-t.net>

freedesktop-unofficial-mirror/xorg__test__xorg-gtest,freedesktop-unofficial-mirror/xorg__test__xorg-gtest,freedesktop-unofficial-mirror/xorg__test__xorg-gtest