dibbler-client accepts malformed IA_PD Prefix option

This is a proposal of fixing bug 308 http://klub.com.pl/bugzilla3/show_bug.cgi?id=308

TClntCfgMgr had methods only to validate ia, ta and addr scopes. I've added methods to validate pd and prefix scopes. logic is basically the same as for validating ia and addr.

I've also added validation on received answers by client (cases where REPLY or REQUEST are received). isValid() call from ClntOptIAPrefix class does the job.

@Tomasz I look forward to getting some feedback from you :)