change the smtpd table protocol

change the smtpd table protocol

Using imsg for the "proc" table (external programs) has proven quite
painful in practice since a lot of smtpd internals (structs, enums,
etc..) have to be kept in sync with the various tables implementations.

Instead, a filter-like protocol for tables decouples the implementations
and allows to write and test tables easily.

The new text-based transport protocol is documented in the (added)
smtpd-tables(7) manpage.

The old imsg protocol is no longer supported and existing tables have to
be converted.  In particular, users of opensmtpd-extras tables will need
install the new opensmtpd-table-* packages.

With lots of suggestions and improvements from gilles and a tweak
from Philipp (philipp+openbsd [at] bureaucracy [dot] de), thanks!

ok gilles