Make hostid consistent in user and kernel space

Make hostid consistent in user and kernel space

If no spl_hostid was set, and no /etc/hostid file existed, the user
and kernel would have different values for the hostid.

The kernel's would be 0.  User space's would depend on the libc
implementation.  On systems with glibc, it would be a generated value,
probably the first 4 bytes of an IP address (see man 3 gethostid and
comments above hostid_read in SPL for details).

This then causes the hostid stored in the labels and in the pool
config not to match the hostid userspace obtains from
get_system_hostid().

Since the kernel has no way to know the libc's generated hostid value,
it serves no purpose for ZFS to use the value.

This patch changes user space's get_system_hostid() to conform to the
kernel's method, first checking for the spl_hostid via sysfs, and then
reading from /etc/hostid directly.

It does not look up spl_hostid_path, because if that is set and the
file it pointed to exists, spl_hostid will reflect its contents.

It eliminates the call to libc's gethostid().

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Ned Bass <bass6@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 #745
Closes #6279