Overhaul use of __pltopt__.m to correctly check for cellstr, not just cell. * scripts/plot/private/__pltopt__.m: Use modern coding conventions. Eliminate unneeded input checking in subfunctions. Remove unused variables. * scripts/plot/contourf.m, scripts/plot/feather.m, scripts/plot/plotmatrix.m, scripts/plot/private/__bar__.m, scripts/plot/private/__quiver__.m, scripts/plot/private/__scatter__.m, scripts/plot/private/__stem__.m, scripts/plot/stairs.m: Use iscellstr, not iscell, to properly validate inputs to __pltopt__.

--- a/scripts/plot/stairs.m
+++ b/scripts/plot/stairs.m
@@ -149,7 +149,7 @@
   have_line_spec = false;
   for i = 1:2:numel (varargin)
     arg = varargin{i};
-    if (ischar (arg) || iscell (arg))
+    if (ischar (arg) || iscellstr (arg))
       [linespec, valid] = __pltopt__ ("stairs", arg, false);
       if (valid)
         have_line_spec = true;