Expect: AX    0000H
ES:DI addr of switcher's service-entry address
Return: AX    0000H if OK, as far as you're concerned
Info: Task switchers call this fn when they are initialized.

Programs that must take special action to coexist with a task
switcher may need to handle this notification fn, but in most
cases, it can be ignored.

ES:SI will contain the task switcher's service entry address.  You can
call that address with AX=0000H if you want some information
about the task switcher that is being initialized.

Return: AX=0000H if you have no objections to this new switcher being
installed.  If a program returns a non-zero value, all tasks may
receive SwNtfyFn 0007H (switcher is exiting).

INT 2fH 4bxxH (switcher functions)
- -

SwNtfy Fn 0000H: A Switcher Is Being Initialized