flow: Use FWW_ flags to wildcard IP DSCP and ECN.

flow: Use FWW_ flags to wildcard IP DSCP and ECN.

It's no longer necessary to maintain a "nw_tos_mask" wildcard member,
since we only care about completely wildcarding the DSCP and ECN
portions of the IP TOS field.  This commit makes that change.  It also
goes a bit further in internally using "tos" to refer to the entire TOS
field (ie, DSCP and ECN).  We must still refer to the DSCP portions as
"nw_tos" externally through OpenFlow 1.0, since that's the convention it
uses.