pgsql: In pg_ctl, report unexpected failure to stat() the postmaster.pi

In pg_ctl, report unexpected failure to stat() the postmaster.pid file.

Any error other than ENOENT is a bit suspicious here, and perhaps should
not be grounds for assuming the postmaster has failed.  For the moment
though, just report it, and don't change the behavior otherwise.  The
intent is mainly to try to determine why we are seeing intermittent
failures in this area on some buildfarm members.

Back-patch to 9.5 where some of these failures have happened.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/b7b5a1899aa3caeef30117f8e36c1f0e68e8847a

Modified Files
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src/bin/pg_ctl/pg_ctl.c |    5 +++++
1 file changed, 5 insertions(+)