Fix regression: renewal and rebinding replies now work properly again

There were two problems:
1) Unicast sends were broken because boost::asio::ip::address_v4() should have
   been provided a host endian argument.  Previously ndhs always used
   broadcast messages, so this was not an issue.
2) The default config.lua did not properly send the assigned ip address
   in the response; it had been broken by the changes that added inform
   support.
Whitelist rt_sigaction and sigaction syscalls.
Should fix a SIGSYS on termination via SIGTERM with glibc.