Fix data loss in UDP source for large packet bursts (+remove IPv4 enforcement)

sumary:
 - use circular buffer instead of simple linear
 - do not read packets when buffer is full, keep them in kernel until more room in buffer is available

Rework UDP source and fix data losses which occurred when burst of packet larger than internal udp_source buffer arrived. In previous version packets were furiously retrieved from system and dropped when buffer was full, instead of utilization in-kernel buffers.

To make it worse, the buffer was linear - it mus be totally emptied to reset writing back to its beginning.
But this probably does not caused much real trouble, not as much as the data loss mentioned earlier.

Two bonus patches remove enforcement of IPv4 when host is resolved, affectively add IPv6 support.