Allow listeners to start before we have an address, close outstanding TCP connections on address change

[cpq]

Main thing here is the update of c->loc when DCHP gives us the IP. It fixes this situation, when we request a web page from a device, but the response comes from IP address 0.0.0.0:

This is how 0.0.0.0 situation happens:

1. Firmware code does mg_mgr_init() and opens HTTP listener mg_http_listen(). The listener connection grabs the local IP address from the interface: c->loc.ip4 = ifp->ip. Since DHCP did not yet gave up the IP, it is 0.
2. DHCP gives up the IP. ifp->ip is now non-0, but all existing connections have c->loc.ip4 as 0.
3. Client makes a request. Listener accepts. Accepted connection inherits c->loc from the listener, which is 0.

So, this situation was there forever, but appeared in my tests with TM4C129 board. This PR fixes up the c->loc when DHCP assigns us the IP.