ZTS: Use ksh and current environment for user_run

The current user_run often does not work as expected. Commands are run
in a different shell, with a different environment, and all output is
discarded.

Simplify user_run to retain the current environment, eliminate eval,
and feed the command string into ksh. Enhance the logging for
user_run so we can see out and err.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11185