iptables_fw_init: fix memory leak

ext_interface is always allocated from either safe_strdup or
get_ext_iface, but is never freed.
Merge pull request #28 from pawitp/pull-1

iptables_fw_init: fix memory leak
fix annoying access denied when re-login

If users submit a new valid token after they have been authenticated,
the gateway will wrongly submit the old token to server, cause the
server responds with 'Tried to login with a token that wasn't
TOKEN_UNUSED' error message and the original valid access right is
also denied by the gateway.
Merge pull request #3 from zhao-gang/master