When an envelope is scheduled for removal or expire, count it as
inflight. The queue sends an ack when it can actually process it.
The max inflight mechanism ensures that the scheduler never sends
envelopes faster than the queue can process.
Merge branch 'master' of ssh.poolp.org:/git/opensmtpd
master uses reallocarray() but it was introduced after OpenBSD 5.5
preventing build of OpenSMTPD -master on OpenBSD -stable.

introduce reallocarray.c and conditionally build it if we detect
the system doesn't provide it
move queue_python to the OpenSMTPD-extras repository