INT 21 - CED (Command EDitor) - INSTALLABLE COMMANDS
        AH = FFH
        AL = subfunction
            00h add installable command
               BL = mode - bit 0 = 1 callable from DOS prompt
                           bit 1 = 1 callable from application
               DS:SI -> CR-terminated command name
               ES:DI -> FAR routine entry point
            01h remove installable command
               DS:SI -> CR-terminated command name
            02h reserved, may be used to test for CED installation
Return: CF clear if successful
        CF set on error
            AX = 01h invalid function
                 02h command not found (subfunction 01h only)
                 08h insufficient memory (subfunction 00h only)
                 0Eh bad data (subfunction 00h only)
        AH = FFh if CED not installed
Note:   CED is a shareware DOS command-line enhancer by Christopher J. Dunford