Remove SIGIO support for input [v5]

This removes all of the SIGIO handling support used for input
throughout the X server, preparing the way for using threads for input
handling instead.

Places calling OsBlockSIGIO and OsReleaseSIGIO are marked with calls
to stub functions input_lock/input_unlock so that we don't lose this
information.

xfree86 SIGIO support is reworked to use internal versions of
OsBlockSIGIO and OsReleaseSIGIO.

v2: Don't change locking order (Peter Hutterer)
v3: Comment weird && FALSE in xf86Helper.c
    Leave errno save/restore in xf86ReadInput
    Squash with stub adding patch (Peter Hutterer)
v4: Leave UseSIGIO config parameter so that
    existing config files don't break (Peter Hutterer)
v5: Split a couple of independent patch bits out
    of kinput.c (Peter Hutterer)

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

config/config.c

@@ -86,10 +86,10 @@ remove_device(const char *backend, DeviceIntPtr dev)
 
     /* Call PIE here so we don't try to dereference a device that's
      * already been removed. */
-    OsBlockSignals();
+    input_lock();
     ProcessInputEvents();
     DeleteInputDeviceRequest(dev);
-    OsReleaseSignals();
+    input_unlock();
 }
 
 void