factual error in the DSIG description in the OT spec.

[Hin-Tak Leung]

Since we are on correcting the spec, here is another issue with
the DSIG description in 

https://www.microsoft.com/typography/otspec/dsig.htm

as well as the ISO/IEC 14496:22 2015 pdf.

The "4. Zero out the file checksum in the head table." 
in the "Format 1: For whole fonts, with either TrueType outlines and/or CFF data:"
 should be removed. i.e. it should read:

<quote>
1 . If there is an existing DSIG table in the font,

    1. Remove DSIG table from font.
    2. Remove DSIG table entry from sfnt Table Directory.
    3 . Adjust table offsets as necessary.
    4. Add the usFlag (reserved, set at 1 for now) to the stream of bytes
</quote>

because there is only one implementation of the signing tool, from Microsoft, for many years,
and that's how it behaves, and that's how all the signed fonts in the past 15+ years look like.
The spec needs to be corrected to match how the one and only signing implementation behaves.

The new DSIG check ( https://github.com/HinTak/Font-Validator/blob/master/DSIGInfo/DSIGInfo.cs ),
which re-implements and replaces the not-opened MS wintrust based mssipotf.dll COM server
in Microsoft's Font Validator and the MS signing tool, behaves like the MS signing tool, not the written spec.

I tried implementing as how the spec was written and was stuck not verifying
known-well-signed fonts for some time, until Cosimo Lupo of
Dalton Maag Ltd tipped me about that error in the spec. The credits go to one of his
(unnamed) colleagues in Dalton Maag for discovering this.


Note also in some rare cases, ttc's hashes are mis-calculated by my new implementation: 
https://github.com/HinTak/Font-Validator/issues/4#issuecomment-161325775
https://github.com/HinTak/Font-Validator/issues/4#issuecomment-193967387