When pthread invokes pthread_exit, it just detach self.

Update license for RT-Thread kernel.

1. Add signal implementation;
2. Change the 'rt_uint8_t' of cmd to 'int';

chang /*@{*/ to /**@{*/ and /*@}*/ to /**@}*/

Skip list is a "random" data structure that in high possibilities it
would get O(log(N)) time complexity in inserting while the old list get
O(N). Forthermore, when set RT_TIMER_SKIP_LIST_LEVEL to 1, it will just
the same as the old double linked list, both in time and space
complexity.

Benchmarks shows that when RT_TIMER_SKIP_LIST_LEVEL is 3, the average
time of random insertion of new timer is about 2 times faster than the
old timer when there are 100 timers and 3 times faster when there are
200 timers.

However, it restores the deprecated funcion rt_system_timer_init. BSPs
must invoke it upon system startup.

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2554 bbd45198-f89e-11dd-88c7-29a3b14d5316

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2506 bbd45198-f89e-11dd-88c7-29a3b14d5316

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2505 bbd45198-f89e-11dd-88c7-29a3b14d5316

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2500 bbd45198-f89e-11dd-88c7-29a3b14d5316

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2263 bbd45198-f89e-11dd-88c7-29a3b14d5316

extern rt_uint8_t rt_current_priority;

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2153 bbd45198-f89e-11dd-88c7-29a3b14d5316

using rt_bool_t instead of rt_err_t

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2150 bbd45198-f89e-11dd-88c7-29a3b14d5316