INT 21 - DOS 1+ - PARSE FILENAME INTO FCB
        AH = 29h
        AL = parsing options
            bit 0: skip leading separators
            bit 1: use existing drive number in FCB if no drive is specified,
                   instead of setting field to zero
            bit 2: use existing filename in FCB if no base name is specified,
                   instead of filling field with blanks
            bit 3: use existing extension in FCB if no extension is specified,
                   instead of filling field with blanks
            bits 4-7: reserved (0)
        DS:SI -> filename string (both '*' and '?' wildcards OK)
        ES:DI -> buffer for unopened FCB
Return: AL = result code
            00h successful parse, no wildcards encountered
            01h successful parse, wildcards present
            FFh failed (invalid drive specifier)
        DS:SI -> first unparsed character
        ES:DI buffer filled with unopened FCB (see AH=0Fh)
Notes:  asterisks expanded to question marks in the FCB
        all processing stops when a filename terminator is encountered
        cannot be used with filespecs which include a path (DOS 2+)
SeeAlso: AH=16h,AH=26h