Fix zdb calling behavior in ztest

Fix zdb calling behavior in ztest

The current zdb calling behaviour is really fragile, and is guaranteed to
segfault if ztest is not installed in either /sbin or /usr/sbin. With this
patch, the ztest will try to call zdb in the following order.

1. Use environmental variable ZDB_PATH if provided.
2. If ztest resides in build tree, guess the in tree zdb path.
3. Just pass zdb to popen and let it search it in PATH.

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3126