Add some logging code for unexpected cases in pgstat.c, particularly being

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.