Add support for PfSense

PfSense is based on FreeBSD and its pf is pretty close to the one
FreeBSD ships, however some structures have different fields and two
offsets had to be fixed.

sshuttle/methods/pf.py

 import os
 import sys
+import platform
 import re
 import socket
 import struct
@@ -335,10 +336,20 @@ class Darwin(FreeBsd):
         return xport.port
 
 
+class PfSense(FreeBsd):
+    RULE_ACTION_OFFSET = 3040
+
+    def __init__(self):
+        self.pfioc_rule = c_char * 3112
+        super(PfSense, self).__init__()
+
+
 if sys.platform == 'darwin':
     pf = Darwin()
 elif sys.platform.startswith('openbsd'):
     pf = OpenBsd()
+elif platform.version().endswith('pfSense'):
+    pf = PfSense()
 else:
     pf = FreeBsd()