Merge branch 'feature-flags'

Feature flags support for ZFS ported from Illumos.  Only minimal
compatibility changes were made where required to accomidate Linux.
For a detailed description of feature flags see original proposal
on zfs-discuss.  They are conceptually very similar to Linux's
ext[234] style of feature flags.

http://lists.freebsd.org/pipermail/freebsd-fs/2011-May/011568.html

NOTE: This branch updates the default pool version for new pools
from 28 to 5000.  Version 28 pools may still be created for
compatibility with Solaris by using the '-o version=28' option.

$ zpool create -o version=28 ...

Existing pools must be manually upgraded using 'zpool upgrade'.

$ zpool upgrade ...

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #778