[openpgp] Version 1 image header bits that MUST be 0

[Paul Wouters <[e-mail]>]

Current text states:

 	The fourth octet of a version 1 image header designates the encoding format of the image.
 	The only currently defined encoding format is the value 1 to indicate JPEG.
 	Image format types 100 through 110 are reserved for private or experimental use.
 	The rest of the version 1 image header is made up of 12 reserved octets, all of which MUST be set to 0.

Where possibly, I'd like to extend claims of "MUST be x" with "and MUST
be ignored if not x". In general, this allows forward compatibility
without old clients dying because of reserved bits now being in use.

I just wanted to run this by the WG to ensure this can be done here as
well. I don't think this would be a security issue, unless we would
want the entire image to fail upon seeing a single unknown bit.

Paul