scsci/sg.h: fix usage of undefined macro HZ

this macro is 100 on all archs, at least in userspace, according
to kernel headers.

add scsi headers scsi.h and sg.h

due to some historical oddity, these are considered libc headers
rather than kernel headers. the kernel used to provide them too, but
it seems modern kernels do not install them, so let's just do the
easiest thing and provide them. stripped-down versions provided by
John Spencer.