Add zfs module feature and property info to sysfs

Add zfs module feature and property info to sysfs

This extends our sysfs '/sys/module/zfs' entry to include feature
and property attributes. The primary consumer of this information
is user processes, like the zfs CLI, that need to know what the
current loaded ZFS module supports. The libzfs binary will consult
this information when instantiating the zfs and zpool property
tables and the pool features table.

This introduces 4 kernel objects (dirs) into '/sys/module/zfs'
with corresponding attributes (files):
  features.runtime
  features.pool
  properties.dataset
  properties.pool

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Don Brady <don.brady@delphix.com>
Closes #7706