INT 21 - F-DRIVER.SYS v1.14+ - GRAB INT 21
        AX = 4BEEh
Return: AX = 1234h if grab successful
           = 2345h if failed (INT 21 grabbed previously)
Notes:  F-DRIVER.SYS is part of the F-PROT virus/trojan protection package by
          Fridrik Skulason
        when called the first time, this function moves the INT 21 monitoring
          code from its original location in the INT 21 chain to be the first
          thing called by INT 21.  This is the mechanism used by F-NET.