Changed decimals to integers in the arcstat script

Changed interval value type from decimal to integer,
because of deprecation warning in Python 3.8 and above.
Also changed kstat values type from decimal to integer,
because all the values are integers.

Fixed behavior of arcstat when run without args.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Bartosz Zieba <bartosz@zieba.pro>
Closes #10132 
Closes #10142

cmd/arcstat/arcstat

@@ -51,7 +51,6 @@ import getopt
 import re
 import copy
 
-from decimal import Decimal
 from signal import signal, SIGINT, SIGWINCH, SIG_DFL
 
 
@@ -139,7 +138,7 @@ if sys.platform.startswith('freebsd'):
 
             name, value = s.name, s.value
             # Trims 'kstat.zfs.misc.arcstats' from the name
-            kstat[name[24:]] = Decimal(value)
+            kstat[name[24:]] = int(value)
 
 elif sys.platform.startswith('linux'):
     def kstat_update():
@@ -158,7 +157,7 @@ elif sys.platform.startswith('linux'):
                 continue
 
             name, unused, value = s.split()
-            kstat[name] = Decimal(value)
+            kstat[name] = int(value)
 
 
 def detailed_usage():
@@ -335,16 +334,8 @@ def init():
         i += 1
 
     argv = sys.argv[i:]
-    sint = Decimal(argv[0]) if argv else sint
-    count = int(argv[1]) if len(argv) > 1 else count
-
-    if len(argv) > 1:
-        sint = Decimal(argv[0])
-        count = int(argv[1])
-
-    elif len(argv) > 0:
-        sint = Decimal(argv[0])
-        count = 0
+    sint = int(argv[0]) if argv else sint
+    count = int(argv[1]) if len(argv) > 1 else (0 if len(argv) > 0 else 1)
 
     if hflag or (xflag and desired_cols):
         usage()