Problem: setcellwidths() may make 'listchars' or 'fillchars' invalid.
Solution: Check the value and give an error. (closesvim/vim#9024)
94358a1e6e
Cherry-pick f_setcellwidths() change from patch 9.0.0036.
Cherry-pick 'ambiwidth' docs update from runtime update 079ba76ae7a7.