pgsql-server: Fix oversight: in case where SIGTERM is received while

Log Message:
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Fix oversight: in case where SIGTERM is received while there are
live backends, the archiver and stats processes never got sent a
kill signal. They'd eventually exit on their own, but not for awhile,
which is a bit annoying when you are trying to replace the executable
file on a platform that doesn't allow removal of busy executables.
Also, tweak main loop logic so that we will perform the background
tasks after select() returns EINTR.

Modified Files:
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pgsql-server/src/backend/postmaster:
postmaster.c (r1.415 -> r1.416)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/postmaster/postmaster.c.diff?r1=1.415&r2=1.416)