Fix vertical cursor motion when there's a display string with newline at EOL.

Fix vertical cursor motion when there's a display string with newline at EOL.

 src/xdisp.c (row_for_charpos_p): New function, with code of
 cursor_row_p, but accepts an additional argument CHARPOS instead
 of using a hardcoded PT.
 (cursor_row_p): Call row_for_charpos_p with 2nd argument PT.
 (row_containing_pos): Call row_for_charpos_p instead of partially
 doing the same.  Fixes cursor positioning under longlines-mode
 when longlines-show-effect includes more than one newline, when
 moving the cursor vertically up.