Added OFPFC_MODIFY_STRICT flow mod command.

Added OFPFC_MODIFY_STRICT flow mod command.

This cleans up the handling of modifying existing flows.  There now exists
OFPFC_MODIFY and OFPFC_MODIFY_STRICT commands, which have the same matching
characteristics to their OFPFC_DELETE* counterparts.  When adding a flow that
already exists, the counters and timers are now reset (ie, its treated as a
new flow).  This commit also adds the "--strict" option to dpctl to allow
strict matching for "mod-flows" and "del-flows".