* Complete overhaul of ppf_mime. Determine the MIME message boundary
using more reliable (albeit more complex) means, and special case
a lot of client behavior to allow verification of a wider variety
of messages. For display, de-code more of the MIME en-coding so that
the messages are much more readable. Use the same tricks to display
decrypted messages in ppf_mime_decrypt.
These changes have several major benefits:
a. Support for PGP/MIME messages generated by well over a dozen MUAs.
b. Support for verifying signatures on attachments, and a clear
indication that attachments are signed (or not).
c. Greatly improved readability. With the exception of text coloring
(URLs, signatures, etc.), 8-bit characters, and some types of
messages sent with format=flowed, messages displayed by the filter
are identical to the display in Alpine.
* For ppf_{decrypt|encrypt|sign|verify} add 'clear' commands so that
nothing is left behind in the "user interface" area between scripts.
For _verify, add a message indicating that we are verifying, along
with a helpful hint about delays caused by auto-key-retrieve.
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