Add snapdev=[hidden|visible] dataset property

The new snapdev dataset property may be set to control the
visibility of zvol snapshot devices.  By default this value
is set to 'hidden' which will prevent zvol snapshots from
appearing under /dev/zvol/ and /dev/<dataset>/.  When set to
'visible' all zvol snapshots for the dataset will be visible.

This functionality was largely added because when automatic
snapshoting is enabled large numbers of read-only zvol snapshots
will be created.  When creating these devices the kernel will
attempt to read their partition tables, and blkid will attempt
to identify any filesystems on those partitions.  This leads
to a variety of issues:

1) The zvol partition tables will be read in the context of
   the `modprobe zfs` for automatically imported pools.  This
   is undesirable and should be done asynchronously, but for
   now reducing the number of visible devices helps.

2) Udev expects to be able to complete its work for a new
   block devices fairly quickly.  When many zvol devices are
   added at the same time this is no longer be true.  It can
   lead to udev timeouts and missing /dev/zvol links.

3) Simply having lots of devices in /dev/ can be aukward from
   a management standpoint.  Hidding the devices your unlikely
   to ever use helps with this.  Any snapshot device which is
   needed can be made visible by changing the snapdev property.

NOTE: This patch changes the default behavior for zvols which
      was effectively 'snapdev=visible'.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1235
Closes #945
Issue #956
Issue #756