clear up statement about trust_mark_owners and trust_mark_issuers for…

[zachmann]

… delegated trust marks

Fixes #123

This relates to delegated trust marks, and the information a TA publishes in trust_mark_issuers for such trust marks.

This includes two changes:

• Changes MUST to SHOULD, since it is generally optional to publish information in trust_mark_issuers, I don't see a reason why it must be a MUST for delegated trust marks.
• Explicitly state that the TA can provide the empty array to accept all Trust Mark Issuers (within the federation) that the Trust Mark Owner has delegated to

[selfissued]

When would it be useful for the Trust Anchor to not publish information about the acceptable Trust Mark Issuers for a Trust Mark identifier? In order words, why does it improve the spec to change the MUST to a SHOULD?

[zachmann]

The MUST only refers to publishing a trust_mark_issuers for trust mark ids that are delegated, i.e. where the TA publishes a trust mark owner in trust_mark_owners.
In the general case it is already optional for a TA to publish information in trust_mark_issuers. In my opinion this also should be the case for delegated trust marks.

[selfissued]

All right. Given that trust_mark_issuers is OPTIONAL, I guess I'm OK with making this a SHOULD.

@rohe and @peppelinux, what do you think?

[selfissued]

Thanks for doing this!