Ensure BackgroundWorker struct contents are well-defined.

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

Commit: b6dd1271281ce856ab774fc0b491a92878e3b501
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2017-04-17T03:23:44Z
Releases: 10.0
Ensure BackgroundWorker struct contents are well-defined.

Coverity complained because bgw.bgw_extra wasn't being filled in by
ApplyLauncherRegister().  The most future-proof fix is to memset the
whole BackgroundWorker struct to zeroes.  While at it, let's apply the
same coding rule to other places that set up BackgroundWorker structs;
four out of five had the same or related issues.

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