zimport.sh: Allow custom pool create options

Allow custom options to be passed to 'zpool create` when creating
a new pool.

Normally zimport.sh is intented to prevent accidentally introduced
incompatibilities so we want the default behavior. However, when
introducing a known incompatibility with a feature flag we need a
way to disable the feature. By adding a line like the following
to the commit message the feature can be disabled allowing the
pool to be compatibile with older versions.

TEST_ZIMPORT_CREATE_OPTIONS="-o feature@encryption=disabled"

• Additionally fix /dev/nul -> /dev/null typo and minor white space formating issues.

• Updated fail function to print a message and exit with 1 for use by the buildbot.

• Silence warnings when zlib_inflate / zlib_default modules don't exist. This can happen when they're build in to the kernel.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6520