Fix 'recenter' when visual-line-mode is turned on

Fix 'recenter' when visual-line-mode is turned on

* src/window.c (Frecenter): Use the same code for GUI and TTY
frames alike; use vmotion only for "initial" frames.  This is
because vmotion doesn't support visual-line-mode.  Rewrite the
'iarg >= 0' case to use move_it_* functions instead of using
vmotion, for the same reason.  Fix the clipping of the argument
value to support scroll-margin in all cases and avoid unwarranted
recentering.  Reported by Milan Stanojević <milanst@gmail.com> in
http://lists.gnu.org/archive/html/help-gnu-emacs/2015-04/msg00092.html,
which see.