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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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/87221867e8b8fedb347e32d3d919e62ae85edc81

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