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/arcstat/arcstat

@@ -54,6 +54,7 @@ import copy
 from decimal import Decimal
 from signal import signal, SIGINT, SIGWINCH, SIG_DFL
 
+
 cols = {
     # HDR:        [Size, Scale, Description]
     "time":       [8, -1, "Time"],
@@ -118,6 +119,26 @@ out = None
 kstat = None
 
 
+if sys.platform.startswith('linux'):
+    def kstat_update():
+        global kstat
+
+        k = [line.strip() for line in open('/proc/spl/kstat/zfs/arcstats')]
+
+        if not k:
+            sys.exit(1)
+
+        del k[0:2]
+        kstat = {}
+
+        for s in k:
+            if not s:
+                continue
+
+            name, unused, value = s.split()
+            kstat[name] = Decimal(value)
+
+
 def detailed_usage():
     sys.stderr.write("%s\n" % cmd)
     sys.stderr.write("Field definitions are as follows:\n")
@@ -148,25 +169,6 @@ def usage():
     sys.exit(1)
 
 
-def kstat_update():
-    global kstat
-
-    k = [line.strip() for line in open('/proc/spl/kstat/zfs/arcstats')]
-
-    if not k:
-        sys.exit(1)
-
-    del k[0:2]
-    kstat = {}
-
-    for s in k:
-        if not s:
-            continue
-
-        name, unused, value = s.split()
-        kstat[name] = Decimal(value)
-
-
 def snap_stats():
     global cur
     global kstat