xfree86: always enable SIGIO on OsVendorInit (#50957)

Drivers call xf86InstallSIGIOHandler() for their fd on DEVICE_ON. That
function does not actually enable the signal if it was blocked to begin
with. As a result, if one vt-switches away from the server (SIGIO is
blocked) and then triggers a server regeneration, the signal remains
blocked and input devices are dead.

Avoid this by always unblocking SIGIO when we start the server.

X.Org Bug 50957 <http://bugs.freedesktop.org/show_bug.cgi?id=50957>

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Keith Packard <keithp@keithp.com>

hw/xfree86/common/xf86Init.c

@@ -964,6 +964,7 @@ OsVendorInit(void)
     }
 #endif
 #endif
+    xf86UnblockSIGIO(0);
 
     beenHere = TRUE;
 }