[libc] Prevent strndup() from reading beyond the end of the string

[libc] Prevent strndup() from reading beyond the end of the string

strndup() may be called on a string which is not NUL-terminated.  Use
strnlen() instead of strlen() to ensure that we do not read beyond the
end of such a string.

Add self-tests for strndup(), including a test case with an
unterminated string.

Originally-fixed-by: Marin Hannache <git@mareo.fr>
Signed-off-by: Michael Brown <mcb30@ipxe.org>