cli/show: add information about which headers were protected

The header-mask member of the per-message crypto object allows a
clever UI frontend to mark whether a header was protected (or not).
And if it was protected, it contains enough information to show useful
detail to an interested user.  For example, an MUA could offer a "show
what this message's Subject looked like on the wire" feature in expert
mode.

As before, we only handle Subject for now, but we might be able to
handle other headers in the future.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
Amended by db: tweaked schemata notation.