Fix zdb -e and zhack thread_init()

Fix zdb -e and zhack thread_init()

This issue was caused by calling `thread_init()` and `thread_fini()`
multiple times resulting in `kthread_key` being invalid.  To resolve
the issue the explicit calls to `thread_init()` and `thread_fini()`
required by the `zpool` command have been moved in to the command.
Consumers such as `zdb` and `zhack` perform the same initialized
through `kernel_init()` and `kernel_fini()`.

Resolving this issue allows multiple additional test cases to
be enabled.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Signed-off-by: Chunwei Chen <tuxoko@gmail.com>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes #4331