networkd: Add DHCPv6 as a configuration option to radv prefixes

The Network section IPv6PrefixDelegation= option takes two new
configuration values, namely "static" and "dhcpv6" in addition
to boolean yes and no values. Static prefixes in IPv6Prefix
sections are used when IPv6PrefixDelegation= option contains
"static", and DHCPv6 is queried for prefixes when the option
contains "dhcpv6". Both DHCPv6 and static prefixes are used when
the option contains a boolean true value. The default value is
false as before, meaning no prefixes are delegated.