修复rt_kprintf输出long long类型数据问题

[src/kservice.c] 修复 rt_vsnprintf 处理 size 和 buf 为 0 极端情况的 bug (对 0 地址写入)
Signed-off-by: NMurphyZhao <d2014zjt@163.com>

Signed-off-by: NMurphyZhao <d2014zjt@163.com>

Update license for RT-Thread kernel.

rename _errno to __rt_errno for conflict in WIN32.

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

Update year info from 2016 to 2017

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

in rt_console_set_device, open new device first and get the new device to _console_device.
我在编写程序时，在device驱动中打印log，但是使用rt_kprintf时，如果_console_device存在非空，那就执行驱动的write函数，但是这是device还没有初始化成功，但是已经提前给_console_device赋值，导致了程序无故fault，以上述修改可避开这样的矛盾。

Commit a6251a43 changed the serial device to check open_flag
instead of flag. So we should change the rt_kprintf counterpart as well.