pkg/zfs should be cgo free

[mmlb]

Issue Description

cgo is currently being used for 2 minor reasons.

1. avoid embedded go pointers into syscall/kernel (the nvlist embedded in the zfs_cmd_t)
2. make use of ioctl, pkg syscall doesn't provide ioctl so we would have to use syscall.Syscall

Acceptance Criteria (how do we know when this is done?)

no use of cgo in pkg/zfs