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.
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