Security framework and objective.metadata

Hi there.
I have been using PyObjC for some time and up until recently i was using
ctypes for the Security framework.

I managed to generate a wrapper for Security by modifying
objective.metadata slightly so that it wouldn't attempt to parse a
linked list recursively in the Security headers. My solution is really
hacky though (basically if this struct contains a member that is this
struct we dont recurse into it).

I'd be happy to work with someone to come up with a cleaner solution as
I'm not very familiar with objective.metadata, clang, etc etc.

Also will send a PR request for objective.metadata command line usage
documentation. It look a few minutes to study the existing structure so
I thought it was worth writing up.

Thanks Ronald and devs.
Mosen.

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