Fix scale clamping in numeric round() and trunc().

The numeric round() and trunc() functions clamp the scale argument to
the range between +/- NUMERIC_MAX_RESULT_SCALE (2000), which is much
smaller than the actual allowed range of type numeric. As a result,
they return incorrect results when asked to round/truncate more than
2000 digits before or after the decimal point.

Fix by using the correct upper and lower scale limits based on the
actual allowed (and documented) range of type numeric.

While at it, use the new NUMERIC_WEIGHT_MAX constant instead of
SHRT_MAX in all other overflow checks, and fix a comment thinko in
power_var() introduced by e54a758d24 -- the minimum value of
ln_dweight is -NUMERIC_DSCALE_MAX (-16383), not -SHRT_MAX, though this
doesn't affect the point being made in the comment, that the resulting
local_rscale value may exceed NUMERIC_MAX_DISPLAY_SCALE (1000).

Back-patch to all supported branches.

Dean Rasheed, reviewed by Joel Jacobson.

Discussion: https://postgr.es/m/CAEZATCXB%2BrDTuMjhK5ZxcouufigSc-X4tGJCBTMpZ3n%3DxxQuhg%40mail.gmail.com

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