INT 21 - DOS 2+ - "RENAME" - RENAME FILE
        AH = 56h
        DS:DX -> ASCIZ filename of existing file (no wildcards, but see below)
        ES:DI -> ASCIZ new filename (no wildcards)
        CL = attribute mask (server call only, see below)
Return: CF clear if successful
        CF set on error
            AX = error code (02h,03h,05h,11h) (see AH=59h)
Notes:  allows move between directories on same logical volume
        does not set the archive attribute (see AX=4300h), which results in
          incremental backups not backing up the file under its new name
        open files should not be renamed
        (DOS 3+) allows renaming of directories
        (DOS 3.1+) wildcards are allowed if invoked via AX=5D00h, in which case
          error 12h (no more files) is returned on success, and both source and
          destination specs must be canonical (as returned by AH=60h).
          Wildcards in the destination are replaced by the corresponding char
          of each source file being renamed.  Under DOS 3.x, the call will fail
          if the destination wildcard is *.* or equivalent.  When invoked via
          AX=5D00h, only those files matching the attribute mask in CL are
          renamed.
        under the FlashTek X-32 DOS extender, the old-name pointer is in DS:EDX
          and the new-name pointer is in ES:EDI (DS must equal ES)
SeeAlso: AX=4301h,AH=60h,AX=5D00h