Add port of FreeBSD 'volmode' property

Add port of FreeBSD 'volmode' property

The volmode property may be set to control the visibility of ZVOL
block devices.

This allow switching ZVOL between three modes:
   full - existing fully functional behaviour (default)
   dev  - hide partitions on ZVOL block devices
   none - not exposing volumes outside ZFS

Additionally the new zvol_volmode module parameter can be used to
control the default behaviour.

This functionality can be used, for instance, on "backup" pools to
avoid cluttering /dev with unneeded zd* devices.

Original-patch-by: mav <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
FreeBSD-commit: https://github.com/freebsd/freebsd/commit/dd28e6bb
Closes #1796
Closes #3438
Closes #6233