Simplify local delivery codepath:

- replace uses of struct batch in the parent with simpler struct delivery.
- replace IMSG_BATCH_* dance with single IMSG_MDA_SESS_NEW.
- make mda assume it delivers to external program over a pipe.
- fork helper process when delivering to maildir or a file.

New feature: upon external mda failure use last line of its output
as an error message.

With input and tests from nicm@.

OK nicm@ gilles@

0 files changed, 0 insertions, 0 deletions