LU-11394 utils: Adjust HOSTID constant

Use constant defined in spl / post-0.8.0 libspl, for HOSTID file.
Also allows get_system_hostid() to be pulled in from ZFS 0.8.x

Test-Parameters: trivial
Signed-off-by: Nathaniel Clark <nclark@whamcloud.com>
Change-Id: Iba70d4f3b7f237260bdc964b28b601deeee81208
Reviewed-on: https://review.whamcloud.com/34278
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Olaf Faaland-LLNL <faaland1@llnl.gov>
Reviewed-by: Oleg Drokin <green@whamcloud.com>