zfs: allow to mount root from a pool not in zpool.cache

[Andriy Gapon]

Currently FreeBSD ZFS kernel code doesn't allow to mount root filesystem on a
pool that is not listed in zpool.cache as only pools from the cache are known to
ZFS at that time.

This patch is an attempt to improve the behavior:
http://people.freebsd.org/~avg/spa_import_rootpool.diff

This could be useful when importing pools that were exported from other systems.
There is a tunable vfs.zfs.rootpool.prefer_cached_config which is set to 1 by
default.  1 means just use a cached pool config if it's found in the cache, 0
means to re-probe disks and read supposedly latest/actual config in any case.

-- 
Andriy Gapon