Fix problems with line-number updates in Follow mode

Fix problems with line-number updates in Follow mode

* src/xdisp.c (redisplay_window): If forced window-start requires
to move a window's point, and the window is under relative
line-number display, force another round of redisplay to update
the relative line numbers.  This fixes follow-mode "redisplay" of
its window group.

* lisp/frame.el: Add display-line-numbers to the list of variables
that should trigger redisplay of the current buffer.