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

etc/Makefile.am

@@ -1,2 +1,2 @@
-SUBDIRS = zfs $(ZFS_INIT_SYSTEMD) $(ZFS_INIT_SYSV) $(ZFS_MODULE_LOAD)
-DIST_SUBDIRS = init.d zfs systemd modules-load.d
+SUBDIRS = zfs sudoers.d $(ZFS_INIT_SYSTEMD) $(ZFS_INIT_SYSV) $(ZFS_MODULE_LOAD)
+DIST_SUBDIRS = init.d zfs systemd modules-load.d sudoers.d

etc/sudoers.d/Makefile.am

@@ -0,0 +1,5 @@
+sudoersddir = $(sysconfdir)/sudoers.d
+sudoersd_DATA = zfs
+
+EXTRA_DIST = \
+	$(top_srcdir)/etc/sudoers.d/zfs

etc/sudoers.d/zfs

@@ -0,0 +1,8 @@
+##
+## Allow any user to run `zpool iostat/status -c smart` in order
+## to read basic SMART health statistics for a pool.
+##
+## CAUTION: Any syntax error introduced here will break sudo.
+##
+
+# ALL ALL = (root) NOPASSWD: /usr/sbin/smartctl -a /dev/[hsv]d[a-z0-9]*