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 ia and addr scopes.

I've also added validation on received answers by client.

@Tomasz I'm looking forward to some feedback from you :)