illumos gate

Adding -B option to ztest which takes the name of a directory which should be
an alternate root directory containing an older version of ztest, zdb, and
libzpool.so.1. For example:

/
home/
chris/
old_ztest/
usr/
bin/
i86/
ztest
amd64/
ztest
sbin/
i86/
zdb
amd64/
ztest
lib/
libzpool.so.1
64/
libzpool.so.1

You would then run

ztest -B /home/chris/old_ztest/

ztest will then invoke the older version of ztest to initialize the pool (so
that the pool is created with the "older" version) and then randomly alternate
between invoking itself (the "newer" version) and the older version each time
it finishes a run or is SIGKILL-ed.

This tests the effect of importing a pool with alternating older and newer
versions of the code. This should continue to work as long as the pool is never
upgraded to the newer version.