(window_min_size_2): New function.

(window_min_size_2): New function.
(window_min_size_1, size_window, Fdisplay_buffer)
(Fsplit_window, adjust_window_trailing_edge): Use it to avoid
windows without mode- or header-lines when window-min-height is
too small.
(size_window): Reset nodelete_p after testing it, following an
earlier note by Kim F. Storm.
(display_buffer): Do not set split_height_threshold to twice the
value of window_min_height to avoid changing the value of a
customizable variable.  Rather explicitly check whether the
height of the window that shall be splitted is at least as large
as split_height_threshold.