Import scsi headers.

Linux's scsi headers are a mix of userspace-facing and kernel-facing
declarations that can't be directly used by userspace.  The glibc
maintainers manually copy-and-pasted these definitions into their own
scsi headers and haven't substantially updated them in 15 years.  musl
has a similar set of definitions in its scsi headers.

This change generates the scsi headers from external/kernel-headers,
using the updated generation script so that the single modified scsi.h
takes the place of the non-modified one.

Change-Id: Ic14d155aa534b0edc1ee686d61113ce5cdf0e6af