Add zgenhostid utility script

Turning the multihost property on requires that a hostid be set to allow
ZFS to determine when a foreign system is attemping to import a pool.
The error message instructing the user to set a hostid refers to
genhostid(1).

Genhostid(1) is not available on SUSE Linux.  This commit adds a script
modeled after genhostid(1) for those users.

Zgenhostid checks for an /etc/hostid file; if it does not exist, it
creates one and stores a value.  If the user has provided a hostid as an
argument, that value is used.  Otherwise, a random hostid is generated
and stored.

This differs from the CENTOS 6/7 versions of genhostid, which overwrite
the /etc/hostid file even though their manpages state otherwise.

A man page for zgenhostid is added. The one for genhostid is in (1), but
I put zgenhostid in (8) because I believe it's more appropriate.

The mmp tests are modified to use zgenhostid to set the hostid instead
of using the spl_hostid module parameter.  zgenhostid will not replace
an existing /etc/hostid file, so new mmp_clear_hostid calls are
required.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #6358
Closes #6379

tests/zfs-tests/include/commands.cfg

@@ -141,6 +141,7 @@ export ZFS_FILES='zdb
     arcstat.py
     dbufstat.py
     zed
+    zgenhostid
     zstreamdump'
 
 export ZFSTEST_FILES='chg_usr_exec