datapath: Allow each vport to have an array of 'port_id's.

In order to allow handlers directly read upcalls from datapath,
we need to support per-handler netlink socket for each vport in
datapath.  This commit makes this happen.  Also, it is guaranteed
to be backward compatible with previous branch.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Thomas Graf <tgraf@redhat.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
ofproto-macros.at: Assert daemon commands creates pidfile before exiting

I observed that "ofproto-dpif - ofproto-dpif-monitor 2" test,
which merely accesses ovsdb, failed in OVS_VSWITCHD_STOP's
"ovs-appctl -t ovs-vswitchd exit".  This commit tries to catch
the problem earlier by sprinkling AT_CHECKs.
ofproto-dpif.at: Fix some races

These races were exposed on my environment by commit c2a77f33.
("tests/ofproto-dpif: Use vlog to test dpif behaviour.")
ofproto-dpif.at: Use OVS_APP_EXIT_AND_WAIT where appropriate

Leftover of commit 0c473314.
("ofproto-dpif.at: Wait for the monitor's pidfile disappears where necessary")
ofproto-dpif.at: Fix races in dpif/dump-flows test
flow.h: Fix a comment typo
ofproto-dpif: Whitespace fixes
dpif-netdev: Remove unused members

Simplify code and update comments after commit 61e7deb1.
("dpif-netdev: Use RCU to protect data.")