Restrict zpool iostat/status -c to search path

Restrict zpool iostat/status -c to search path

zpool iostat/status -c is supposed to be restricted
by its search path, but currently isn't. To prevent
arbitrary scripts from being executed, disallow '/'
from commands.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #6353
Closes #6359