bug#27734: 26.0.50; display-line-numbers: allow to set alternate face for multiples of N
When displaying line numbers using the new `display-line-numbers'
feature, we could add a face to be used for each line number that is a
multiple of some number N, which could be specified by the user.
display-line-numbers-alternate (interger or nil to disable)
line-number-alternate (alternative face)
When used with line-number-current-line, line-number-current-line
obviously takes precedence.