Added test if ZRE_DISCOVERY_PORT/udp is open

Hi all,

While using zyre, i found out that there is no detection, if port 5670/udp is open or not. With following changes, where i added check, if  ZRE_DISCOVERY_PORT/udp port is open

One can test it with following code:
```
  zctx_t *ctx = zctx_new();
  zyre_t *node1 = zyre_new(ctx);
  printf("Node pointer is %p \n",node1);
```
when user runs it and port is closed, user would see and node1 pointer will be null:
mvala@vala ~ $ /home/mvala/test/bin/test/zt-test-zyre
14-03-18 11:55:31 E: Discovery is not possible !!!
14-03-18 11:55:31 E: Make sure that port 5670/udp is open !!!
Node pointer is (nil) 

if port is open
mvala@vala ~ $ /home/mvala/test/bin/test/zt-test-zyre
Node pointer is 0x9ab0b0 

Ciao

Martin