* lisp/calc/calc-prog.el: Switch to new method of detecting end of kbd macro

* lisp/calc/calc-prog.el: Switch to new method of detecting end of kbd macro

'read-char' will no longer return -1 as of
ac82baea1c41ec974ad49f2861ae6c06bda2b4ed. This switches to a cleaner
method of detecting whether the end of a keyboard macro has been
reached.

* lisp/calc/calc-prog.el (calc--at-end-of-kmacro-p): New function.
(calc-kbd-skip-to-else-if): Use the function.

Co-authored-by: Stefan Monnier <monnier@iro.umontreal.ca>
(cherry picked from commit df3e0bcbdbcfe907d7572b5561dd2bf9c3715a4a)