RBIL61 - INT 2F - DOS 3.3, DOS 5.0 APPEND - HOOK INT 21 {undocumented}

INT 2F U - DOS 3.3, DOS 5.0 APPEND - HOOK INT 21                                
	AX = B703h
	ES:DI -> INT 21 handler APPEND should chain to
Return: ES:DI -> APPEND's INT 21 handler
Note:	each invocation of this function toggles a flag which APPEND uses to
	  determine whether to chain to the user handler or the original
	  INT 21
SeeAlso: AX=B700h,AX=B706h