net: combat buffer bloat

Set tx_queue_len to 50 (FreeBSD default) from 1000 to reduce latencies.  Set
tcp_ecn to 1 to force enable Explicit Congestion Notification.  These two
changes should make a positive impact network performance.  Possible drop in
throughput with 1 gbit and faster networks.
net: restore tcp_ecn default

Forcing ECN breaks some Cisco routers with SPI firewall enabled.