lib/string.c: implement strdup() and strndup() in terms of memdup_nul()

lib/string.c: implement strdup() and strndup() in terms of memdup_nul()

With the addition of memdup_nul(), strdup() and strndup() can be
implemented as one-liners.

While not required by POSIX or C, do keep the behaviour of gracefully
accepting a NULL source and simply return NULL.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>