linux: Report supported user features to the kernel

Following commit (''netlink: Do not enforce alignment of last Netlink
attribute''), signal the ability to receive unaligned Netlink messages
to the datapath to enable utilization of zerocopy optimizations.

Opening a datapath is now done by issueing a OVS_DP_CMD_SET in order
to overwrite previously set user features.

Signed-off-by: Thomas Graf <tgraf@redhat.com>
Acked-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>