Add ENETUNREACH to NET_ERRS

We shouldn't come up with a fatal error because of a ENETUNREACH when
trying to contact the DNS server. Although this error shouldn't happen
either.

Fixes #89.

sshuttle/ssnet.py

@@ -54,7 +54,8 @@
 
 NET_ERRS = [errno.ECONNREFUSED, errno.ETIMEDOUT,
             errno.EHOSTUNREACH, errno.ENETUNREACH,
-            errno.EHOSTDOWN, errno.ENETDOWN]
+            errno.EHOSTDOWN, errno.ENETDOWN,
+            errno.ENETUNREACH]
 
 
 def _add(l, elem):