fix for osx10.7 to define symbol ENONET as EHOSTDOWN

Building zyre on osx10.7/clang gives the following error:

zre_udp.c:250:18: error: use of undeclared identifier 'ENONET'
    ||  errno == ENONET
                 ^
1 error generated.
make[2]: *** [zre_udp.lo] Error 1

I've redefined ENONET symbol to fix this. Let me know if there's a better way to do this.