ip route: enable per-route ecn settings via 'features' option

This permits to selectively enable explicit congestion notification via
the routing table.

If this ecn feature is not set, the kernel will use the tcp_ecn sysctl
to decide wheter to use ECN when establising a TCP connection.

At the time of this writing, the kernel supports ecn and allfrags, but
allfrags is of dubious value and not implemented here.

Example:

ip route change 192.168.2.0/24 dev eth0 features ecn

Signed-off-by: Florian Westphal <fw@strlen.de>