unix: add IPMI interface

Add structs and constants for the Linux IPMI system interface.

Change-Id: I100c402bfdf9dd72b541b612c8783248c35bd16b
Reviewed-on: https://go-review.googlesource.com/c/sys/+/752624
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Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: David Chase <drchase@google.com>