Set UDP packet size limit to 512 bytes

[a1k0n]

Workaround for #363. Substantially more reliable on OS X with this, though the reason packets are lost remains elusive.

Not honestly sure whether this should be user-configurable, or to change the default in GNU radio, or what. What do you think @csete ?

[csete]

Thanks for the patch.

I'd like to avoid user configurable parameters as much as possible. If we wrap this change into

#ifdef GQRX_OS_MACX
    d_sink = gr::blocks::udp_sink::make(sizeof(short), "localhost", 7355, 512);
#else
    d_sink = gr::blocks::udp_sink::make(sizeof(short), "localhost", 7355);
#endif

then we don't break anything for existing users. We can in time test and see if it works just as well on linux and use the same packet size. If you resubmit with the ifdef or something similar I will merge it.

[a1k0n]

Ok, #ifdef'd. Please take another look.

[csete]

Excellent, thanks.