ofproto: Break subrules out of "struct rule" into new "struct facet".

ofproto: Break subrules out of "struct rule" into new "struct facet".

Until now, there have been three types of struct rule:

   - An OpenFlow rule with wildcards, that may have exact-match "subrules".

   - An OpenFlow rule without wildcards.

   - A subrule that has no wildcards.

This is at best confusing.  At worst, it leads to bugs.

This commit changes the situation to:

   - "struct rule" represents an OpenFlow rule, with or without wildcards.

   - "struct facet" is what was called a subrule before.

This commit is intended to be more or less the minimal patch necessary to
implement this change.  Followup commits will further improve the code.