add initial support for 2nd wheel & 4-5 mouse buttons

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+This document describes an extension to the int33 API to allow users to detect
+if mouse interrupts come from a relative-input device (like a standard mouse)
+or an absolute-input device (like a tablet, or an emulator like VirtualBox).
+
+---------------------------------------------------------------------
+Changes in the original INT 33h functions:
+
+INT 33/000C - Define User Interrupt Routine
+INT 33/0014 - Exchange User Interrupt Routines
+Bitfields for mouse call mask:
+Bit(s)	Description
+ 0-6	same as Table 03171 (as in int33.lst)
+ 7      (for vertical wheel movement -- seee wheelapi.txt)
+ 8      absolute mouse event
+ 9-15	unused
+Notes:	when the user interrupt routine is called, bit 8 of CX indicates that
+          the x, y coordinates passed in CX, DX come from an absolute pointing
+          device (and therefore that the mickey counts in SI, DI may be zero or
+          virtualized).
+        bit 8 will not be set unless the user also sets bits 8 in the event mask
+          passed to int33/000c or int33/0014. However, setting or clearing bit 8
+          in the event mask shall have no other effect.