chapters/software-stack: Simplify os_strcat() implementation.

The solution provided to `os_strcat()` was too esotheric and not suited
to be used by students wanting go learn how to implement `strcat()`.
The current solution is a bit more verbose, but still includes the
previous approac as a nice-to-know gimmick.

Signed-off-by: Teodor Dutu <[email protected]>

chapters/software-stack/libc/drills/tasks/common-functions/solution/src/os_string.c

@@ -23,16 +23,30 @@ char *os_strcpy(char *dest, const char *src)
  return dest;
 }
 
-/* TODO 12: Implement strcat(). */
+/* TODO 25: Implement os_strcat(). */
 char *os_strcat(char *dest, const char *src)
 {
  char *rdest = dest;
 
  while (*dest)
  dest++;
 
+ while (*src) {
+ *dest = *src;
+ dest++;
+ src++;
+ }
+
+ /*
+ * A more difficult to understand, but shorter and smarter implementation is below.
+ * The ++ operator is evaluated last, after checking the condition of the loop, so this condition can be
+ * seen as simply *dest = *src followed by dest++ and src++ in the loop body, which is equivalent to the loop
+ * above.
+ */
+ /*
  while (*dest++ = *src++)
  ;
+ */
 
  return rdest;
 }