Fix oversight: in case where SIGTERM is received while there are

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.