patch 9.1.0990: Inconsistent behavior when changing cmdheight

patch 9.1.0990: Inconsistent behavior when changing cmdheight

Problem:  Inconsistent behavior when changing cmdheight by resizing the
          topframe through wincmds and dragging laststatus. Changing
	  cmdheight by resizing the topframe does not trigger OptionSet.
Solution: Consolidate logic for changing the cmdheight, set the option
	  value to handle side-effects (Luuk van Baal)

closes: #16359

Signed-off-by: Luuk van Baal <luukvbaal@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>