(calc-convert-units): Make units a local variable.

(calc-convert-units): Make units a local variable.
(calc-num-units, calc-den-units): New variables.
(calc-explain-units, calc-explain-units-rec): Replace variables
num-units and den-units by declared variables.
(math-cu-unit-list): New variable.
(math-build-units-table, math-compare-unit-names)
(math-convert-units, math-convert-units-rec): Replace variable
unit-list by declared variable.
(math-fbu-base, math-fbu-entry): New variables.
(math-find-base-units, math-find-base-units-rec): Replace variables
base and entry by declared variables.
(math-which-standard): New variable.
(math-to-standard-units, math-to-standard-rec): Replace variable
which-standard by declared variable.
(math-fcu-u): New variable.
(math-find-compatible-unit, math-find-compatible-unit-rec):
Replace variable u by declared variable.
(math-cu-new-units, math-cu-pure): New variables.
(math-convert-units, math-convert-units-rec): Replace variables
new-units and pure by declared variables.
(math-try-cancel-units): New variable.
(math-simplify-units-quotient): Replace variable try-cancel-units by
declared variable.