[kernel]Change 'rt_timer_timeout_hook' function to 'rt_timer_enter_hook' and add 'rt_timer_exit_hook' hook function.

Update license for RT-Thread kernel.

[kernel] add RT_USED to INIT_EXPORT, so that the initialization function is not deleted when it is linked.

Address the comments from armink@rt-thread

Some non-gnu toolchains don't support gnu C extended keyword "typeof", like MDK (if no "gnu" option) and IAR, in this case we can provide list element manipulation macro functions to iterate over the list.

Remove the restriction of RT_USING_POSIX for waitqueue structure.