remove unnecessary codes

part of kernel/kernel.c
kernel/start.asm
kthread: add: prio property
arch/x86/schedule.c: fix coding style
add comments with doxygen format
task_*: print space instead of \t when clear messages
new: a syscall for getting timer ticks

hide global variable _g_timer_ticks
rename pid to tid
bootsect: kernel image requires more larger buffer
merge functions of lib.h itoa.c mem.c to string.c
add more types into types.h
move initialization of kthread[0] to c_main()
keyboard.c: remove initialization message
move arch-dependent parts to arch folder
introduce 'current' macro
new shell command: stat
rewrite enable_int and disable_int
gdt.c: remove ununsed codes
x86/screen.c: rewrite screen_puts
Makefile: update rules
fix all compile warnings