Add soreceive_dgram(9), an optimized socket receive function for use by

Add soreceive_dgram(9), an optimized socket receive function for use by
datagram-only protocols, such as UDP.  This version removes use of
sblock(), which is not required due to an inability to interlace data
improperly with datagrams, as well as avoiding some of the larger loops
and state management that don't apply on datagram sockets.

This is experimental code, so hook it up only for UDPv4 for testing; if
there are problems we may need to revise it or turn it off by default,
but it offers *significant* performance improvements for threaded UDP
applications such as BIND9, nsd, and memcached using UDP.

Tested by:	kris, ps