INT 2F - SHARE - INSTALLATION CHECK
        AX = 1000h
Return: AL = 00h  not installed, OK to install
             01h  not installed, not OK to install
             FFh  installed
BUGS:   values of AL other than 00h put DOS 3.x SHARE into an infinite loop
          (08E9: OR  AL,AL
           08EB: JNZ 08EB) <- the buggy instruction (DOS 3.3)
        values of AL other than described here put PCDOS 4.00 into the same
          loop (the buggy instructions are the same)
Notes:  supported by OS/2 v1.3+ compatibility box, which always returns AL=FFh
        if DOS 4.01 SHARE was automatically loaded, file sharing is in an
          inactive state (due to the undocumented /NC flag used by the autoload
          code) until this call is made
        DOS 5.0 chains to the previous handler if AL <> 00h on entry
SeeAlso: INT 21/AH=52h