Handle EACCES errors from kevent() better.

While registering for postmaster exit events, we have to handle a couple
of edge cases where the postmaster is already gone.  Commit 815c2f09
missed one: EACCES must surely imply that PostmasterPid no longer
belongs to our postmaster process (or alternatively an unexpected
permissions model has been imposed on us).  Like ESRCH, this should be
treated as a WL_POSTMASTER_DEATH event, rather than being raised with
ereport().

No known problems reported in the wild.  Per code review from Tom Lane.
Back-patch to 13.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/3624029.1602701929%40sss.pgh.pa.us

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