Fix bug #11261 with cursor movement in a continued line when header line is present.

Fix bug #11261 with cursor movement in a continued line when header line is present.

 src/xdisp.c (string_buffer_position_lim): Limit starting position to BEGV.
 (set_cursor_from_row): If called for a mode-line or header-line
 row, return zero immediately.
 (try_cursor_movement): If inside continuation line, don't back up
 farther than the first row after the header line, if any.  Don't
 consider the header-line row as "partially visible", even if
 MATRIX_ROW_PARTIALLY_VISIBLE_P returns non-zero.