pgsql: Rename pgbench min/max to least/greatest, and fix handling of do

Rename pgbench min/max to least/greatest, and fix handling of double args.

These functions behave like the backend's least/greatest functions,
not like min/max, so the originally-chosen names invite confusion.
Per discussion, rename to least/greatest.

I also took it upon myself to make them return double if any input is
double. The previous behavior of silently coercing all inputs to int
surely does not meet the principle of least astonishment.

Copy-edit some of the other new functions' documentation, too.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/7a622b2731db5d0f6db8a3d0af88177f96d1cb2e

Modified Files
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doc/src/sgml/ref/pgbench.sgml | 25 +++++++++--------
src/bin/pgbench/exprparse.y | 8 +++---
src/bin/pgbench/pgbench.c | 64 ++++++++++++++++++++++++++++++++-----------
src/bin/pgbench/pgbench.h | 4 +--
4 files changed, 67 insertions(+), 34 deletions(-)