Update HWMP Proactive code and mesh route flags.
Update HWMP Proactive code and mesh route flags.

* Modified hwmp_recv_preq:
    o cleaned up code, removed rootmac variable because preq->origaddr
      is the root when we recevie a Proactive PREQ;
    o Modified so that a PREP in response of a Proactive PREQ is unicast,
      a PREP is ALWAYS unicast;
* Modified hwmp_recv_prep:
    o Before we mark a route to be valid we should remove the discovery
      flag and then mark it valid in such a way we wont lose the isgate flag;

Approved by: adrian

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