[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.

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

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

Use signed int as array index is unsecure.

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.

If two timer will timeout at the same tick, the one started later will
be called later. I've tested the patch on simulator and it _seems_ OK.
Reported-by: xdzy on the forum and delin17 <delin17@qq.com>

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2493 bbd45198-f89e-11dd-88c7-29a3b14d5316

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2484 bbd45198-f89e-11dd-88c7-29a3b14d5316

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2323 bbd45198-f89e-11dd-88c7-29a3b14d5316

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git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2149 bbd45198-f89e-11dd-88c7-29a3b14d5316

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git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2108 bbd45198-f89e-11dd-88c7-29a3b14d5316

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2045 bbd45198-f89e-11dd-88c7-29a3b14d5316

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2005 bbd45198-f89e-11dd-88c7-29a3b14d5316

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@1998 bbd45198-f89e-11dd-88c7-29a3b14d5316

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@1715 bbd45198-f89e-11dd-88c7-29a3b14d5316

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@1503 bbd45198-f89e-11dd-88c7-29a3b14d5316

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@1494 bbd45198-f89e-11dd-88c7-29a3b14d5316

add IAR compiler version check in weak; add rt_malloc_page/rt_free_page for application module; fix the software timer thread stack issue.

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@1183 bbd45198-f89e-11dd-88c7-29a3b14d5316

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git-svn-id: https://rt-thread.googlecode.com/svn/trunk@1117 bbd45198-f89e-11dd-88c7-29a3b14d5316