rt_ssize_t can give negative error code, which follows the unix style correctly
1. Change the type of cmd to 'int'; 2. Remove RT_LWIP_USING_RT_MEM macro;
git-svn-id: https://rt-thread.googlecode.com/svn/trunk@1865 bbd45198-f89e-11dd-88c7-29a3b14d5316