Make Temp Buffer Resize Mode less intrusive (Bug#1806).

Make Temp Buffer Resize Mode less intrusive (Bug#1806).

* window.c (Fsplit_window_internal): Handle only Qt value of
Vwindow_combination_limit separately.
(Qtemp_buffer_resize): New symbol.
(Vwindow_combination_limit): New default value.  Rewrite
doc-string.
* cus-start.el (window-combination-limit): Add new optional
values.
* window.el (temp-buffer-window-show)
(window--try-to-split-window): Obey new values of
window-combination-limit.
(split-window): Test window-combination-limit for t instead of
non-nil.
(display-buffer-at-bottom): New buffer display action function.
* help.el (temp-buffer-resize-regexps): New option.
(temp-buffer-resize-mode): Rewrite doc-string.
(resize-temp-buffer-window): Obey temp-buffer-resize-regexps.
Don't resize reused window.  Suggested by Glen Morris.