Re: avoid multiple hard links to same WAL file after a crash

Here is an attempt at creating something that can be back-patched.  0001
simply replaces calls to durable_rename_excl() with durable_rename() and is
intended to be back-patched.  0002 removes the definition of
durable_rename_excl() and is _not_ intended for back-patching.  I imagine
0002 will need to be held back for v16devel.

I think back-patching 0001 will encounter a couple of small obstacles.  For
example, the call in basic_archive won't exist on most of the
back-branches, and durable_rename_excl() was named durable_link_or_rename()
before v13.  I don't mind producing a patch for each back-branch if needed.

-- 
Nathan Bossart
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