Use /2 and 2* instead of >>1 and <<1 which are tricky with signed types

Today's compilers generate shift instructions to perform division and
multiplications by powers of 2 [1]. `(x >> 1)` looks straightforward enough, but
if x is signed the code will fail when x < 0. The compiler knows better: use
`x / 2`.

That's why we have code like this:

    (long)((long_u)Rows >> 1))

instead of the cleaner version that generates the same or better machine code:

    Rows / 2

[1] http://goo.gl/J4WpG7

src/eval.c

@@ -7162,7 +7162,7 @@ find_internal_func (
    * Find the function name in the table. Binary search.
    */
   while (first <= last) {
-    x = first + ((unsigned)(last - first) >> 1);
+    x = first + (last - first) / 2;
     cmp = STRCMP(name, functions[x].f_name);
     if (cmp < 0)
       last = x - 1;