Under new theory of operation wherein postmaster forks children

Under new theory of operation wherein postmaster forks children
immediately, we will fork a child even if the database state does not
permit connections to be accepted (eg, we are in recovery mode).
The child process will correctly reject the connection and exit as
soon as it's finished collecting the connection request message.
However, this means that reaper() must be prepared to see child
process exit signals even while it's waiting for startup or shutdown
process to finish.  As was, a connection request arriving during a
database recovery or shutdown would cause postmaster abort.