pgsql: Add some logging code for unexpected cases in pgstat.c,

Log Message:
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Add some logging code for unexpected cases in pgstat.c, particularly being
unable to read a stats file for reasons other than ENOENT, and having to reset
last_statrequest because it's later than current time in the collector.
Not clear if this will shed any light on the "pgstat wait timeout" business,
but it seems like a good idea in general.

In passing, do some message-style-police work on recently-added
pgstat_reset_shared_counters code.

Modified Files:
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    pgsql/src/backend/postmaster:
        pgstat.c (r1.201 -> r1.202)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c?r1=1.201&r2=1.202)