Specifying parent modes (e.g. prog-mode) in :enabled-for-modes and
:disabled-for-modes keys in dotspacemacs-line-numbers catches derived
modes (e.g. c-mode) as well.

Fix bug where an empty :disabled-for-modes and a non-empty :enabled-for-modes
enabled line numbers everywhere, instead of only in modes specified in
:enabled-for-modes. (see #8482)

Add auxiliary function spacemacs/derived-mode-p, similar to derived-mode-p but
takes a major-mode as an argument instead of using the current major-mode.