Change parameters list for sys_thread_new (see "task #7252 : Create sys_thread_new_ex...

Change parameters list for sys_thread_new (see "task #7252 : Create sys_thread_new_ex()"). Two new parameters have to be provided: a task name, and a task stack size. For this one, since it's platform dependant, you could define the best one for you in your lwipopts.h. For port maintainers, you can just add these new parameters in your sys_arch.c file, and but it's not mandatory, use them in your OS specific functions.