lib: implement strnstr()

lib: implement strnstr()

Implement library function strnstr().
Implement strstr() using strnstr().
Sort the includes.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
[jf: replace <stdint.h> by <limits.h>, folded from next patch]
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>