Factor Linux specific pieces out of libspl. Reviewed-by: Ryan Moeller <ryan@ixsystems.com> Reviewed-by: Sean Eric Fagan <sef@ixsystems.com> Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Signed-off-by: Matt Macy <mmacy@FreeBSD.org> Closes #9336