networkd: Add support for ipv6 privacy extension

networkd: Add support for ipv6 privacy extension

This patch add support for ipv6 privacy extensions.

The variable  /proc/sys/net/ipv6/conf/<if>/use_tempaddr

can be changed via the boolean

IPv6PrivacyExtensions=[yes/no/prefer-temporary]

When true enables privacy extensions, but prefer public addresses over
temporary addresses.
prefer-temporary  prefers temporary adresses over public addresses.
Defaults to false.

[Match]
Name=enp0s25

[Network]
IPv6PrivacyExtensions=prefer-temporary