OpenPGP: implement pgp_find_blob()
OpenPGP: implement pgp_find_blob()

Replace the "one-trick-pony" pgp_do_iswritable() with a more generic
function returning the blob matching the passed tag.

This way we can get rid of the one-line function pgp_blob_iswritable() too.
comparisons like these can be done in the caller.

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