Handle EHOSTDOWN, ENETDOWN.

Someone on the mailing list reported getting these.

Also centralize the list of these errors, so we don't have different parts
of the code supporting a different subset of them.  Now just use
ssnet.NET_ERRS.

server.py

@@ -130,7 +130,7 @@ class DnsProxy(Handler):
         try:
             self.sock.send(self.request)
         except socket.error, e:
-            if e.args[0] in [errno.ECONNREFUSED, errno.EHOSTUNREACH]:
+            if e.args[0] in ssnet.NET_ERRS:
                 # might have been spurious; try again.
                 # Note: these errors sometimes are reported by recv(),
                 # and sometimes by send().  We have to catch both.
@@ -145,7 +145,7 @@ class DnsProxy(Handler):
         try:
             data = self.sock.recv(4096)
         except socket.error, e:
-            if e.args[0] in [errno.ECONNREFUSED, errno.EHOSTUNREACH]:
+            if e.args[0] in ssnet.NET_ERRS:
                 # might have been spurious; try again.
                 # Note: these errors sometimes are reported by recv(),
                 # and sometimes by send().  We have to catch both.

ssnet.py

@@ -42,6 +42,10 @@ cmd_to_name = {
 }
 
 
+NET_ERRS = [errno.ECONNREFUSED, errno.ETIMEDOUT,
+            errno.EHOSTUNREACH, errno.ENETUNREACH,
+            errno.EHOSTDOWN, errno.ENETDOWN]
+
 
 def _add(l, elem):
     if not elem in l:
@@ -151,9 +155,7 @@ class SockWrapper:
             elif e.args[0] == errno.EISCONN:
                 # connected successfully (BSD)
                 self.connect_to = None
-            elif e.args[0] in [errno.ECONNREFUSED, errno.ETIMEDOUT,
-                               errno.EHOSTUNREACH, errno.ENETUNREACH,
-                               errno.EACCES, errno.EPERM]:
+            elif e.args[0] in NET_ERRS + [errno.EACCES, errno.EPERM]:
                 # a "normal" kind of error
                 self.connect_to = None
                 self.seterr(e)