Expect: AX    0001H
BX    ID of the session that is about to be suspended
ES:DI addr of switcher's service-entry address
Return: AX    0000H if OK to suspend, as far as you're concerned
0001H if you object to the switch
Info: Task switchers call this fn when they are about to perform a
session switch.  Your application can prevent the switch or
perform any special cleanup needed, then allow the switch to take
place.  When called, interrupts are enabled and all DOS fns can
be used.

BX will contain the session ID of the session that is about to be
switched out.  Global TSRs may wish to take special actions for
each session.

ES:SI will contain the task switcher's service entry address.  Use this
address if you need to call service fns such as SwSrvcFn 0001H
(test memory region) to determine how you feel about being
switched out.

Return: AX=0000H if you have no objection to being switched out. If any
task returns a non-zero value, the session will not be switched
out and all tasks may receive SwNtfyFn 0004H (session is active).

INT 2fH 4bxxH (switcher functions)
- -

SwNtfy Fn 0001H: Can Switcher Suspend Your Session?