pgsql: Use floor() not rint() when reducing precision of fractional

Log Message:
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Use floor() not rint() when reducing precision of fractional seconds in
timestamp_trunc, timestamptz_trunc, and interval_trunc(). This change
only affects the float-datetime case; the integer-datetime case already
behaved like truncation instead of rounding. Per gripe from Mario Splivalo.

This is a pre-existing issue but I'm choosing not to backpatch, because
it's such a corner case and there have not been prior complaints. The
issue is largely moot anyway given the trend towards integer datetimes.

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