Use derived-mode-p for functions to also work for derived modes

[meliache]

In my case I recently started using dirvish, which is an enhanced dired-mode, and I was surprised that crux-open-with didn't work, but I fixed it by using derived-mode-p.

Then, I searched for other occurrences of major-mode and also replaced there

(equal major-mode '<mode>)

by

(derived-mode-p '<mode>)

I'm still not sure whether I should also do that in crux-cleanup-buffer-or-region, where in checking the membership in crux-untabify-sensitive-modes and crux-indent-sensitive-modes, derived modes are note considered. These variables are customizable and I think not considering derived modes there adds control. But here I would be happy on feedback.

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[bbatsov]

Thanks!