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.

scsi/scsi.h has been modified from the 3.18 headers to put #define
__KERNEL__  ... #endif around the sections that other libc
implementations don't expose.  scsi/{scsi_ioctl,sg}.h are the original
upstream headers.  The cleaned-up versions of these headers are
equivalent to the same headers in glibc and musl (though much more
up-to-date).

Change-Id: I3395f7433d91c4c661bd1b88d392b1b11f1ad557
Signed-off-by: Greg Hackmann <ghackmann@google.com>