This adjusts commit 82233ce7 so that the
postmaster does not exit until all its child processes have exited, even
if the 5-second timeout elapses and we have to send SIGKILL.  There is no
great value in having the postmaster process quit sooner, and doing so can
mislead onlookers into thinking that the cluster is fully terminated when
actually some child processes still survive.

This effect might explain recent test failures on buildfarm member hamster,
wherein we failed to restart a cluster just after shutting it down with
"pg_ctl stop -m immediate".

I also did a bit of code review/beautification, including fixing a faulty
use of the Max() macro on a volatile expression.

Back-patch to 9.4.  In older branches, the postmaster never waited for
children to exit during immediate shutdowns, and changing that would be
too much of a behavioral change.