[h323plus] Some issues with h323pdu.cxx

Hello!

Please correct me if I'm wrong, but

1. It looks like there is a small mistake in h323pdu.cxx which doesn't allow to build h323plus without H.235 being enabled.

2. I've found that 'H323SetAliasAddress(const PString & _name, H225_AliasAddress & alias, int tag)' doesn't allow to set AliasAddress of type h323_ID if it looks like dialedDigits even if _name begins with 'h323:'. For example, if I specify _name = 'h323:12345', alias tag will all the same be dialedDigits because it looks like them. I suggest a small change to H323SetAliasAddress, because now it produces unpredictable results.

Thanks,

Denis