There was an error in `linum-on` when `dotspacemacs-line-numbers` was set to
`'relative`.
- new function `spacemacs//linum-backward-compabitility` to test for old
supported values.
- Use an :around advice for `linum-on` instead of redefining it.
- move linum init time config to `:init` section of `use-package`
- fix relative linum initialization by testing if `dotspacemacs-line-numbers` is
a list first.
d5d80eb1a1