INT 2F - DOS 5+ DOSKEY - INSTALLATION CHECK
        AX = 4800h
Return: AL = nonzero if installed (DOS 5.0 and 6.0 return AX=AA02h)
            ES = segment of DOSKEY resident portion
Note:   DOSKEY chains if AL is not 00h or 10h on entry
SeeAlso: AX=4800h"PCED",AX=4810h

.