this generally went unnoticed, as the tunnel usually terminates right
before we exit anyway. however, if multiple Channels are synced, it may
become visible.
this is a "shotgun" implementation, where the main loop just reaps all
unclaimed children.
arguably, it would be cleaner if each socket actually tracked its own
process, but getting synchronous kills+waits right is tricky, so we
continue to pretend that there is no process as far as the socket layer
is concerned.
poll()/select() are not restartable, so they need EINTR handling now
that SIGCHLD is actually delivered.
so far, we checked for M_DEAD only in loops over messages. but we should
have checked srec->msg uses as well. this would make the code a mess, so
instead call back from the drivers when messages are expunged, so we can
reset the pointers.
the only case where this really matters so far is the flag setting loop,
which may cause the concurrent expunge of not yet handled messages to be
detected by the maildir driver.
this tests only the common case of the far side being async - adding
100% instead of 50% to the runtime of the test to cover a corner case
didn't seem worth it.
