Move the filter infrastructure forward.

The smtp process now writes incoming messages on a socket sent by the
mfa, using buffering and flow-control.  The idea is that the mfa sets
a chain of filters up for each incoming message, and send the input
stream to the smtp process.
Merge branch 'master' into filter-smtp
Merge branch 'master' into filter-smtp
improve
better
better
better
getting closer
Merge branch 'master' into filter-smtp
filter parser
better
Merge branch 'master' into filter-smtp
add default empty filter chain is needed
cleanup
Merge branch 'master' into filter-smtp

Conflicts:
	smtpd/filter_api.c
	smtpd/smtp_session.c
Merge branch 'master' into filter-smtp

Conflicts:
	smtpd/filters/Makefile.inc
Merge branch 'master' into filter-smtp
better
Merge branch 'master' into filter-smtp

Conflicts:
	smtpd/filter_api.c
	smtpd/mfa.c
	smtpd/mfa_session.c
	smtpd/smtpd.h
Merge branch 'master' into filter-smtp

Conflicts:
	smtpd/filter_api.c
	smtpd/filters/Makefile.inc
	smtpd/mfa_session.c