Properly apply DIX locking to input events in NetBSD-specific DDX servers.

This thread-based DIX locking approach using input_lock() and
input_unlock() has been available since X.Org 1.19.0:
https://gitlab.freedesktop.org/xorg/xserver/-/commit/30ac7567980a1eb79d084a63e0e74e1d9a3af673

All xfree86 DDX implementations have switched to using these new
DIX locks instead of the old OsBlockSIGIO() / OsReleaseSIGIO() methods:
https://gitlab.freedesktop.org/xorg/xserver/-/commit/6a5a4e60373c1386b311b2a8bb666c32d68a9d99

The xquartz DDX (an official X.Org server implementation) also uses
these locks for input events:
https://gitlab.freedesktop.org/xorg/xserver/-/commit/7d6ebf3f4e4d517bb846d15a5deb131da19a267c

This should resolve occasional SIGSEGVs in the Xsun server caused
by NULL pointer dereferences around the event queue handlers
under heavy load.

Should be pulled up to netbsd-10.

Add "Emulate3Buttons" support to the X68k Xorg based monolithic server.

Based on "Emulate3Buttons" functions in the xf86-input-mouse driver,
with various refactoring and cleanups:
- rewrite a magic three-dimensional array for a finate state machine
with proper named structures etc.
- separate 3 button emulation functions from other stuff

See my post on port-x68k@ (and tech-x11@) for more details:
https://mail-index.netbsd.org/port-x68k/2021/02/05/msg000074.html