Add *pipe-EXTENSION*, ext. *mime-counter-evidence* (Bob Tennent)..

Add *pipe-EXTENSION*, ext. *mime-counter-evidence* (Bob Tennent)..

Outsource and generalize the fetching of external handlers into
the new mimepart_get_handler() function and add a *pipe-EXTENSION*
mechanism that takes precedence before *pipe-CONTENT/SUBCONTENT*.

While here, go the first step in extending *mime-counter-evidence*
and convert it to a value option, let bit 2 mean "don't forget the
counter-evidence, use it to lookup the respective
*pipe-CONTENT/SUBCONTENT*.

Reported and requested by Bob Tennent (rdt AT cs DOT queensu DOT ca).

While doing this i realized and fixed two further bugs:

- The `[Directly address message only to display this]' code path
  would have printed a plain text part regardless of an installed
  pipe handler.

- Pass through ASYNC flag correctly.