net: don't reset connections that overflow the listen backlog queue

The networking stack has a queue of connections that were established but
not yet accepted.  Connections that exceed the length of the queue are dropped.

The stack has two ways of dropping a connection: sending an RST or silently.
The default, sending an RST, will affect the sender, while dropping the
packet silently will allow the sender to retry and hopefully allow the
connection to be established.

Change the default to silently dropping the packet, as it is more friendly
to users, and matches Linux.

Seen with iperf -P 16, which quickly overwhelms the listen backlog and causes
a few streams to fail.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>