pkg/term: use IoctlGetWinsize/IoctlSetWinsize from golang.org/x/sys/unix
pkg/term: use IoctlGetWinsize/IoctlSetWinsize from golang.org/x/sys/unix

Use unix.IoctlGetWinsize and unix.IoctlSetWinsize instead of manually
reimplementing them.

Signed-off-by: Tobias Klauser <9249c549618448e4c699e8032ba6c7f0d7fd4b7f@distanz.ch>

moby/term