Handle invalid options in arc_summary

If an invalid option is provided to arc_summary.py we handle any error
thrown from the getopt Python module and print the usage help message.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6983

cmd/arc_summary/arc_summary.py

@@ -977,9 +977,15 @@ def main():
     global show_tunable_descriptions
     global alternate_tunable_layout
 
-    opts, args = getopt.getopt(
-        sys.argv[1:], "adp:h", ["alternate", "description", "page=", "help"]
-    )
+    try:
+        opts, args = getopt.getopt(
+            sys.argv[1:],
+            "adp:h", ["alternate", "description", "page=", "help"]
+        )
+    except getopt.error as e:
+        sys.stderr.write("Error: %s\n" % e.msg)
+        usage()
+        sys.exit(1)
 
     args = {}
     for opt, arg in opts:
@@ -991,7 +997,7 @@ def main():
             args['p'] = arg
         if opt in ('-h', '--help'):
             usage()
-            sys.exit()
+            sys.exit(0)
 
     Kstat = get_Kstat()
 
@@ -1006,7 +1012,7 @@ def main():
         except IndexError:
             sys.stderr.write('the argument to -p must be between 1 and ' +
                              str(len(unSub)) + '\n')
-            sys.exit()
+            sys.exit(1)
     else:
         pages = unSub