Don't modify argv[] in user tools

argv[] gets modified during string parsing for input arguments. This
is reflected in the live process listing. Don't do that.

Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: DHE <git@dehacked.net>
Closes #7760