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

cmd/zpool/zpool.d/lsblk

@@ -38,16 +38,15 @@
 #   DISC-ZERO  discard zeroes data
 #
 # If the script is run as just 'lsblk' then print out disk size, vendor,
-# model number and serial number.
+# and model number.
 
 
 helpstr="
 label:	Show filesystem label.
 model:	Show disk model number.
-serial:	Show disk serial number.
 size:	Show the disk capacity.
 vendor:	Show the disk vendor.
-lsblk:	Show the disk size, vendor, model number, and serial number."
+lsblk:	Show the disk size, vendor, and model number."
 
 script=$(basename "$0")
 
@@ -57,7 +56,7 @@ if [ "$1" = "-h" ] ; then
 fi
 
 if [ "$script" = "lsblk" ] ; then
-	list="size vendor model serial"
+	list="size vendor model"
 else
 	list=$(echo "$script" | tr '[:upper:]' '[:lower:]')
 fi