Use bind() to check if a local address is still valid.

Use bind() to check if a local address is still valid.

Some platforms don't support getifaddrs(). We use this to check if the
local address of a socket is still available on any network interface.
Instead, try to bind() a new socket to the same address (but port 0) as
existing sockets. If it returns EADDRNOTAVAIL, we know that this address
is no longer valid.