ofproto: Make NXAST_RESUBMIT take header modifications into account.

ofproto: Make NXAST_RESUBMIT take header modifications into account.

Until now, the NXAST_RESUBMIT action has always looked up the original
flow except for the updated in_port.  This commit changes the semantics to
instead look up the flow as modified by any preceding actions that affect
it, e.g. if OFPAT_SET_VLAN_VID precedes NXAST_RESUBMIT, then NXAST_RESUBMIT
now looks up the flow with the modified VLAN, not the original (as well as
the modified in_port).

Also, document how NXAST_RESUBMIT is supposed to work.

Suggested-by: Paul Ingram <paul@nicira.com>