Make idle backends exit if the postmaster dies.

Letting backends continue to run if the postmaster has exited prevents
PostgreSQL from being restarted, which in many environments is
catastrophic.  Worse, if some other backend crashes, we no longer have
any protection against shared memory corruption.  So, arrange for them
to exit instead.  We don't want to expend many cycles on this, but
including postmaster death in the set of things that we wait for when
a backend is idle seems cheap enough.

Rajeev Rastogi and Robert Haas