OpenZFS 8962 - zdb should work on non-idle pools

Currently `zdb` consistently fails to examine non-idle pools as it
fails during the `spa_load()` process. The main problem seems to be
that `spa_load_verify()` fails as can be seen below:

    $ sudo zdb -d -G dcenter
    zdb: can't open 'dcenter': I/O error

    ZFS_DBGMSG(zdb):
    spa_open_common: opening dcenter
    spa_load(dcenter): LOADING
    disk vdev '/dev/dsk/c4t11d0s0': best uberblock found for spa dcenter. txg 40824950
    spa_load(dcenter): using uberblock with txg=40824950
    spa_load(dcenter): UNLOADING
    spa_load(dcenter): RELOADING
    spa_load(dcenter): LOADING
    disk vdev '/dev/dsk/c3t10d0s0': best uberblock found for spa dcenter. txg 40824952
    spa_load(dcenter): using uberblock with txg=40824952
    spa_load(dcenter): FAILED: spa_load_verify failed [error=5]
    spa_load(dcenter): UNLOADING

This change makes `spa_load_verify()` a dryrun when ran from
`zdb`. This is done by creating a global flag in zfs and then setting
it in `zdb`.

Authored by: Pavel Zakharov <[email protected]>
Reviewed by: George Wilson <[email protected]>
Reviewed by: Matthew Ahrens <[email protected]>
Reviewed by: Andy Stormont <[email protected]>
Approved by: Dan McDonald <[email protected]>
Ported-by: Tim Chase <[email protected]>
Signed-off-by: Tim Chase <[email protected]>

OpenZFS-issue: https://illumos.org/issues/8962
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/180ad792
Closes #7459

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