Hiding legacy-display parts

[Daniel Kahn Gillmor]

Now that notmuch can handle and interpret protected subject lines, it
should also avoid forcing the user to look at "legacy display" parts
that some MUAs (notably enigmail) copies of the protected headers that
are intended to be rendered only by legacy clients -- clients capable
of decryption but which don't understand how to handle protected
headers.

This series starts off with known-broken tests and ends by fixing the
problem by "fast-forwarding" through those MIME parts that are
reliably detectable as legacy-display parts.

Comments and feedback welcome!

         --dkg