fill zero when a file is created
fix wrong offset handling in page cache write

1. add ioctl in VfsFile
2. add TIOC{G,S}PGRP in TTY
3. add ioctl syscall

1. add pgid field in LightProcess
2. add {get,set}pgid syscall
3. make kill/wait respect to pgid

Because ioctl is not ready yet, the busybox will fail to start.
This will be fixed in next commit.