pkg/osutil: use functionality from x/sys/unix
pkg/osutil: use functionality from x/sys/unix

Use unix.Unmount instead of manually wrapping SYS_UMOUNT2.

Use unix.IoctlSetPointerInt instead of manually wrapping SYS_IOCTL. This
also allows to use FS_IOC_SETFLAGS instead of manually defining it for
each GOARCH.

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

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