Interval literal rounding bug(?) and patch.

I think it's a bug that these 3 different ways of writing 0.7 seconds
produce different results from each other on HEAD.

head=# select interval '0:0:0.7', interval '@ 0.70 secs', interval '0.7 seconds';
interval | interval | interval
-------------+-----------------+-----------------
00:00:00.70 | 00:00:00.699999 | 00:00:00.699999
(1 row)

The attached patch will make all of those output "00:00:00.70" which.

Postgres 8.3 tended to output the "00:00:00.70" like this patch, I believe
because it didn't default to HAVE_INT64_TIMESTAMP like HEAD is. The patch
seems to pass the existing regression tests.

Does this seem reasonable?

Ron