* lisp/calc: Fix a few issues introduced by lexical scoping

* lisp/calc: Fix a few issues introduced by lexical scoping

Fix a few places I missed, where we incorrectly used lexical scoping on a var
that needed dynamic scoping.
These were detected thanks to a bit of footwork by Mattias Engdegård!

* lisp/calc/calc-ext.el (math-read-big-lines): Declare as dynbound.
(math-read-big-bigp): Bind it inside a `let`.
* lisp/calc/calc-graph.el (math-arglist): Declare as dynbound.
* lisp/calc/calc-map.el (math-arglist): Declare as dynbound.
* lisp/calc/calc-misc.el (math-trunc-prec): Declare as dynbound.
(math-trunc): Bind it inside a `let`.
(math-floor-prec): Declare as dynbound.
(math-floor): Bind it inside a `let`.
* lisp/calc/calc-nlfit.el (calc-curve-varnames, calc-curve-coefnames):
Declare as dynbound.
* lisp/calc/calc-sel.el (math-comp-sel-tag): Declare as dynbound.
* lisp/calc/calcsel2.el (calc-sel-reselect): Declare as dynbound.