Refactor X68k input device handling to remove SIGIO based event processing.

The keyboard and pointer input events are now processed directly
using fd event notifications registered via DIX SetNotifyFd(),
instead of the legacy SIGIO-based asynchronous I/O.

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.

TAB/space cleanup.

Remove clause 3 and 4 from X68k server files written by Yasushi Yamasaki.

I've finally contacted with Yamasaki-san and he kindly answered
"no objection to change them to 2 clause license" in private mail.
Diffs to be committed have also been confirmed by him.
We appreciate all his contribution to early NetBSD/x68k.

Port the ancient Xsun based monolithic MD X68k server to X11R7, like Xnest.

Most diffs (API changes from xfree to xorg and some WARNSfy)
are mechanical. Confirmed working on X68030 and XM6i.

See my post on port-x68k@ for details:
http://mail-index.netbsd.org/port-x68k/2014/02/27/msg000043.html