(sh-set-shell): Pass args no-query-flag

(sh-set-shell): Pass args no-query-flag
and insert-flag to executable-set-magic.
Don't call executable-set-magic if insert-flag is nil.
(sh-mode): Always call sh-set-shell, but use insert-flag arg
to control whether to call executable-set-magic.
No need to set comment-start-skip directly here.

(sh-mode): Parse the interpreter from the file text.