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/plotmatrix.m
+++ b/scripts/plot/plotmatrix.m
@@ -129,7 +129,7 @@
   parent = get (bigax, "parent");
   for i = 1 : nargin - 1
     arg = varargin{i};
-    if (ischar (arg) || iscell (arg))
+    if (ischar (arg) || iscellstr (arg))
       [linespec, valid] = __pltopt__ ("plotmatrix", varargin{i}, false);
       if (valid)
         have_line_spec = true;