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>

include/input.h

@@ -714,4 +714,13 @@ extern _X_HIDDEN void input_constrain_cursor(DeviceIntPtr pDev, ScreenPtr screen
                                              int *out_x, int *out_y,
                                              int *nevents, InternalEvent* events);
 
+static inline void input_lock(void) {
+}
+
+static inline void input_unlock(void) {
+}
+
+static inline void input_force_unlock(void) {
+}
+
 #endif                          /* INPUT_H */