unix: add IOCTL_MEI_* constants

Change-Id: I4efea3d97ae21ea81758e492cabc980b48e5d4c2
Reviewed-on: https://go-review.googlesource.com/c/sys/+/718440
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>