zpool iostat/status -c improvements

zpool iostat/status -c improvements

Users can now provide their own scripts to be run
with 'zpool iostat/status -c'. User scripts should be
placed in ~/.zpool.d to be included in zpool's
default search path.

Provide a script which can be used with
'zpool iostat|status -c' that will return the type of
device (hdd, sdd, file).

Provide a script to get various values from smartctl
when using 'zpool iostat/status -c'.

Allow users to define the ZPOOL_SCRIPTS_PATH
environment variable which can be used to override
the default 'zpool iostat/status -c' search path.

Allow the ZPOOL_SCRIPTS_ENABLED environment
variable to enable or disable 'zpool status/iostat -c'
functionality.

Use the new smart script to provide the serial command.

Install /etc/sudoers.d/zfs file which contains the sudoer
rule for smartctl as a sample.

Allow 'zpool iostat/status -c' tests to run in tree.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #6121
Closes #6153