networkd: Add Router Advertisement variables

networkd: Add Router Advertisement variables

Add variables for enabling Router Advertisements, router lifetime as
well as managed and other information flags indicating use of DHCPv6.
Add configuration of default router preferences as defined in RFC 4191.

IPv6PrefixDelegation in the [Network] section has to be set in order
to enable prefix delegation. The rest of the prefix delegation values
are stored in the [IPv6PrefixDelegation] section. The host will act as
a default router if it is given a non-zero lifetime with
RouterLifetimeSec. Managed and OtherInformation booleans set the level
of DHCPv6 support, and the RouterPreference configures the router's
preference between low, medium and high. Words 'normal' and 'default'
are added as synonyms for 'medium' just to make configuration simpler.

This adds a section like the following to .network configuration files:
[Network]
IPv6PrefixDelegation=true

[IPv6PrefixDelegation]
RouterLifetimeSec=2000
Managed=false
OtherInformation=true
RouterPreference=medium