Isolate code specific to Linux in cmd/

Use sys.platform to choose the correct implementation of functions and
values of variables for the platform being run on.

Reword some comments to avoid describing implementation details in the
wrong places.

Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9561

cmd/arc_summary/arc_summary2

@@ -55,6 +55,30 @@ from subprocess import Popen, PIPE
 from decimal import Decimal as D
 
 
+if sys.platform.startswith('linux'):
+
+    def load_kstats(namespace):
+        """Collect information on a specific subsystem of the ARC"""
+
+        kstat = 'kstat.zfs.misc.%s.%%s' % namespace
+        path = '/proc/spl/kstat/zfs/%s' % namespace
+        with open(path) as f:
+            entries = [line.strip().split() for line in f][2:] # Skip header
+        return [(kstat % name, D(value)) for name, _, value in entries]
+
+    def load_tunables():
+        basepath = '/sys/module/zfs/parameters'
+        tunables = {}
+        for name in os.listdir(basepath):
+            if not name:
+                continue
+            path = '%s/%s' % (basepath, name)
+            with open(path) as f:
+                value = f.read()
+            tunables[name] = value.strip()
+        return tunables
+
+
 show_tunable_descriptions = False
 alternate_tunable_layout = False
 
@@ -77,15 +101,6 @@ def get_Kstat():
     of the same name.
     """
 
-    def load_kstats(namespace):
-        """Collect information on a specific subsystem of the ARC"""
-
-        kstat = 'kstat.zfs.misc.%s.%%s' % namespace
-        path = '/proc/spl/kstat/zfs/%s' % namespace
-        with open(path) as f:
-            entries = [line.strip().split() for line in f][2:] # Skip header
-        return [(kstat % name, D(value)) for name, _, value in entries]
-
     Kstat = {}
     Kstat.update(load_kstats('arcstats'))
     Kstat.update(load_kstats('zfetchstats'))
@@ -917,18 +932,6 @@ def _tunable_summary(Kstat):
     global show_tunable_descriptions
     global alternate_tunable_layout
 
-    def load_tunables():
-        basepath = '/sys/module/zfs/parameters'
-        tunables = {}
-        for name in os.listdir(basepath):
-            if not name:
-                continue
-            path = '%s/%s' % (basepath, name)
-            with open(path) as f:
-                value = f.read()
-            tunables[name] = value.strip()
-        return tunables
-
     tunables = load_tunables()
     descriptions = {}