Compatibility: 5.0+
Expects: AX    4B02H  (INT 2fH)
BX    0000H (must be 0)
ES    0000H (must be 0)
Returns: AX    0000H if task switcher is installed
ES:BX address of service-function handler (if switcher installed)
0000:0000 if switched is not installed
Info: This determines whether a task switcher has been installed and
obtains the service function address (used for accessing switcher

It is possible that more than one task switcher is active (for
instance, if DOSSHELL is executed while in a Windows command
prompt session).  To see if that's the case, use SwSrvcFn 0000H
and examine the pfPrevSwitcher field of the SwVersionRec
structure returned by that call.

Task switchers that intercept this call can enable interrupts can
use any DOS fns.

- -

INT 2fH 4B02H: Is Switcher Installed? / Get Service Addr