Rename rnd to round in I.1

At first, I assumed that this is "rand" function, not "round" function,
and was confused why exactly is it a problem that this returns
different values. While it's easy to notice when actually reading the
code, this slows down the comprehension, and bad function naming is
not point of that particular guideline.

CppCoreGuidelines.md

@@ -1223,12 +1223,12 @@ Correctness. Assumptions not stated in an interface are easily overlooked and ha
 
 Controlling the behavior of a function through a global (namespace scope) variable (a call mode) is implicit and potentially confusing. For example:
 
-    int rnd(double d)
+    int round(double d)
     {
-        return (rnd_up) ? ceil(d) : d;    // don't: "invisible" dependency
+        return (round_up) ? ceil(d) : d;    // don't: "invisible" dependency
     }
 
-It will not be obvious to a caller that the meaning of two calls of `rnd(7.2)` might give different results.
+It will not be obvious to a caller that the meaning of two calls of `round(7.2)` might give different results.
 
 ##### Exception