wifinina: fix concurrency issues with multiple sockets

wifinina driver was not handling concurrent socket connections
correctly.  When trying to make multiple socket connections, the sockfd
returned by the first Socket() call would be the same sockfd returned by
subsequent Socket() calls.  The problem is the sockfd returned by
Socket() isn't used until Connect() (or Appect()).  This would result in
Connect() trying to use the same sockfd for multiple connections,
causing wifinina fw to lock up.

The solution in this PR is to create a new sockfd space managed by the
driver that gives the app a unique, safe sockfd for each connection.
The real underlying sock fd returned by fw is set on Connect() (or
Accept()), and mapped back to the apps sockfd using a map:

    sockets map[int]*Socket // keyed by sockfd

Where Socket has a reference to the fw sock:

    type Socket struct {
            protocol        int
            clientConnected bool
            laddr           netip.AddrPort // Set in Bind()
            raddr           netip.AddrPort // Set in Connect()
            sock                           // Device socket, as returned from w.getSocket()
    }

netdev/netdev.go

@@ -39,6 +39,7 @@ var (
 	ErrNoMoreSockets        = errors.New("No more sockets")
 	ErrClosingSocket        = errors.New("Error closing socket")
 	ErrNotSupported         = errors.New("Not supported")
+	ErrInvalidSocketFd      = errors.New("Invalid socket fd")
 )
 
 // Duplicate of non-exported net.errTimeout