networkd: Add IPv6Prefix sections to the network configuration files

networkd: Add IPv6Prefix sections to the network configuration files

Support zero or more [IPv6Prefix] sections in the network configuration
files. Each section can have one Prefix=<subnet>[/<prefixlength>]
option, with the preferred and valid lifetimes specified by
ValidLifetimeSec and PreferredLifetimeSec measured in seconds, and
with onlink and address autoconfiguration booleans specified by
OnLink and AddressAutoconfiguration variables.

This adds a section like the following to .network configuration files:
[IPv6Prefix]
Prefix=2001:db8:dead:beef::/64
OnLink=true
AddressAutoconfiguration=true
ValidLifetimeSec=1500
PreferredLifetimeSec=1000