Add Strlen(), a strlen(3) that panics if string is stupid long and returns unsigned.

First batch of changes to use it to suppress warnings.

src/topten.c

@@ -117,7 +117,7 @@ formatkiller(
         /*FALLTHRU*/
     case KILLED_BY:
         (void) strncat(buf, killed_by_prefix[how], siz - 1);
-        l = strlen(buf);
+        l = Strlen(buf);
         buf += l, siz -= l;
         break;
     }