ofproto-dpif: Expose datapath capability to ovsdb.

ofproto-dpif: Expose datapath capability to ovsdb.

The patch adds support for fetching the datapath's capabilities
from the result of 'check_support()', and write the supported capability
to a new database column, called 'capabilities' under Datapath table.

To see how it works, run:
 # ovs-vsctl -- add-br br0 -- set Bridge br0 datapath_type=netdev
 # ovs-vsctl -- --id=@m create Datapath datapath_version=0 \
     'ct_zones={}' 'capabilities={}' \
     -- set Open_vSwitch . datapaths:"netdev"=@m

 # ovs-vsctl list-dp-cap netdev
 ufid=true sample_nesting=true clone=true tnl_push_pop=true \
 ct_orig_tuple=true ct_eventmask=true ct_state=true \
 ct_clear=true max_vlan_headers=1 recirc=true ct_label=true \
 max_hash_alg=1 ct_state_nat=true ct_timeout=true \
 ct_mark=true ct_orig_tuple6=true check_pkt_len=true \
 masked_set_action=true max_mpls_depth=3 trunc=true ct_zone=true

Signed-off-by: William Tu <u9012063@gmail.com>
Tested-by: Greg Rose <gvrose8192@gmail.com>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>
---
v5:
    Add improved documentation from Ben and
    fix checkpatch error (tab and line 79 char)
v4:
    rebase to master
v3:
    fix 32-bit build, reported by Greg
    travis: https://travis-ci.org/williamtu/ovs-travis/builds/599276267
v2:
	rebase to master