dhcp6-client: Function for enabling/disabling IA_NA request

dhcp6-client: Function for enabling/disabling IA_NA request

Add function to enable/disable IA_NA address requests. Internally
handle the request as a bit mask and add IA_PD prefix delegation
to the same bit mask instead of having a separate boolean. Thus
the calling code can set requests for prefix and address delegation
separately. This is handy when supporting RFC 7084.

Add a check in the code that at least something is requested from
the server in Managed mode. By default request IA_NA addresses from
the DHCPv6 server. Although a value has been defined for IA_TA,
temporay IA_TA addresses are not yet requested.