coap_io.[c,h] -- minimize changes to github/master branch

Minimize changes to github/master branch to allow for easy migration with github/develop branch.

• #include <strings.h> is not guarded with a #ifdef HAVE_STRING_H. add this guard. Note: strings.h is needed for strerrno used in our additional code.
• delete the following unused code (testing fragments not needed):

warn("not using contiki settings, sending on udp\n");

/*  int socket = udp_comm_open_send_socket(AF_INET);
  if (socket>0){
  	udp_comm_send(data, datalen, socket, dst, 5683);
	udp_comm_close_socket(socket);
  }
 return datalen;
*/

• revert insertion of empty lines
• Move our added code into a separate {.c,.h}-File: The coded added by us is not a change of existing code but adds two new functions, namely
  • udp_comm_open_send_socket
  • udp_comm_close_socket
  • udp_comm_send