zsys_interface: use default value from an environment variable if supplied.

On Mac OS X, it appears that INADDR_BROADCAST (255.255.255.255) cannot be bound to. According to the man page:

> In addition broadcast packets may be sent (assuming the underlying network supports this) by using a reserved ``broadcast address''; this address is network interface dependent.

If I use this patch and specify `ZSYS_INTERFACE=en0 make check` then the beacon tests pass on Mac OS X. Without specifying an interface, 255.255.255.255 is chosen which returns an error code EADDRNOTAVAIL causing an assert.