Initial implementation of UDP socket interface for CSP.

Since UDP, like CSP, doesn't have any notion of connections, we need some way to determine which destination (at a UDP level) to send CSP responses to.  We cheat and send outgoing packets to whichever host we last received a packet from.  This should work pretty well as long as we only have a single client and/or packet handling on the interface is single-threaded. Our usual case.

@mconst-nanosatisfi want to take a look, see if I screwed up anything obvious?