Fix kqueue support under debugger on macOS.

Fix kqueue support under debugger on macOS.

While running under a debugger, macOS's getppid() can return the
debugger's PID.  That could cause a backend to exit because it falsely
believed that the postmaster had died, since commit 815c2f09.

Continue to use getppid() as a fast postmaster check after adding the
postmaster's PID to a kqueue, to close a PID-reuse race, but double
check that it actually exited by trying to read the pipe.  The new check
isn't reached in the common case.

Reported-by: Alexander Korotkov <a.korotkov@postgrespro.ru>
Discussion: https://postgr.es/m/CA%2BhUKGKhAxJ8V8RVwCo6zJaeVrdOG1kFBHGZOOjf6DzW_omeMA%40mail.gmail.com