Compatibility: 3.0+
Expects: AX    1680H
Returns: AL    status: 00H = system supports Idle Calls
else= system does not support program suspension
Info: DOS programs and TSRs should use this while waiting for user
input.  It announces to the system (DOS or Windows) that the
program is willing to give up the current timeslice.

Before calling this the first time, you should use DOS fn 35H to
check that the vector for INT 2FH is non-zero.

The system does not suspend your program unless there is another
task ready to run; most often, control will immediately return.

You should place a call to this fn inside your idle loop,
whenever waiting for keyboard input or polling the mouse, etc.

Notes: This fn is also described (with the same parameters and
functionality) as part of the DPMI specification.

- -

INT 2fH 1680H: Program Idle (Release Timeslice)