Fix bug #18384 with incorrect reporting of row number by posn-col-row.

Fix bug #18384 with incorrect reporting of row number by posn-col-row.

 lisp/subr.el (posn-col-row): Revert the change from commit
 r99634.2.576 eliz@gnu.org-20101113210758-8ml5kibjtza5ysmb, which
 was inadvertently merged from emacs-23 release branch in r102428
 monnier@iro.umontreal.ca-20101118035414-yvlg7k7dk4k4l3q, and
 introduced an off-by-one error in the reported row when there is a
 header line.

 src/dispnew.c (buffer_posn_from_coords): Fix an off-by-one error in
 the reported row in the case of a window with a header line, by
 improving on the fix committed in r106022
 eliz@gnu.org-20111008105850-ht4tvsayohvr1kjc.