upstream: Fix two race conditions in sshd relating to SIGHUP:

1. Recently-forked child processes will briefly remain listening to
  listen_socks. If the main server sshd process completes its restart
  via execv() before these sockets are closed by the child processes
  then it can fail to listen at the desired addresses/ports and/or
  fail to restart.

2. When a SIGHUP is received, there may be forked child processes that
  are awaiting their reexecution state. If the main server sshd
  process restarts before passing this state, these child processes
  will yield errors and use a fallback path of reading the current
  sshd_config from the filesystem rather than use the one that sshd
  was started with.

To fix both of these cases, we reuse the startup_pipes that are shared
between the main server sshd and forked children. Previously this was
used solely to implement tracking of pre-auth child processes for
MaxStartups, but this extends the messaging over these pipes to include
a child->parent message that the parent process is safe to restart. This
message is sent from the child after it has completed its preliminaries:
closing listen_socks and receiving its reexec state.

bz#2953, reported by Michal Koutný; ok markus@ dtucker@

OpenBSD-Commit-ID: 7df09eacfa3ce13e9a7b1e9f17276ecc924d65ab
