Make WaitLatch's WL_POSTMASTER_DEATH result trustworthy; simplify callers.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: f40022f1adaeff85b01d98fea38cf5aa16814aa7
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2012-05-10T18:34:53Z
Releases: 9.2.0
Make WaitLatch's WL_POSTMASTER_DEATH result trustworthy; simplify callers.

Per a suggestion from Peter Geoghegan, make WaitLatch responsible for
verifying that the WL_POSTMASTER_DEATH bit it returns is truthful (by
testing PostmasterIsAlive).  Then simplify its callers, who no longer
need to do that for themselves.  Remove weasel wording about falsely-set
result bits from WaitLatch's API contract.

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