fix socket buffer size detection

[garabik]

In find_max_buffer_size, if the maximum network buffer size is under 1MB, the loop will reach and return zero with unpredictable consequences (e.g. my python coredumps).

This implements "proper" way of finding the buffer size by interval halving. Note that I am not quite able to test it thoroughly, all my linux installations (and some Windows I managed to get access to) seem to happily access any (big) buffer size, regardless of net.core.{r,w}mem_max.