INT 2F - DOS 5+ TASK SWITCHER - ALLOCATE SWITCHER ID                            
	AX = 4B03h
	ES:DI -> task switcher entry point (see #02819)
Return: AX = 0000h
	BX = switcher ID (0001h-000Fh), or 0000h if no more available
Notes:	if a task switcher has determined that it is the first to be loaded, it
	  must allocate an identifier for itself and provide this function to
	  all subsequent task switchers; if it is not the first to be loaded,
	  it must call this function to allocate an ID.	 The switcher ID is
	  used as the high four bits of all session identifiers to ensure
	  unique session IDs.
	if no more switcher IDs are available, the new task switcher making the
	  call must terminate or disable itself
	the task switcher providing the identifiers may call the new task
	  switcher's entry point as needed
	this call is available from within DOSSHELL even if the task switcher
	  is not installed
	this function is supported by PCIBM PC Tools v8+ CPTASK, but appears to
	  always return an ID of 0000h
SeeAlso: AX=4B02h,AX=4B04h