introduce curr_taskinfo(offset) built-in function

curr_taskinfo will return unsigned int value of (task_struct + offset),
for offset, user may need to get offset by gdb outside ktap session.

ktap will support direct access memory in future, like Dtrace.

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@gmail.com>
add fetch_bytes argument into curr_task_info library function

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@gmail.com>