networkd: fix default value of DHCPv6Client

The configuration of networkd has a DHCPv6Client setting in its
[IPv6AcceptRA] section, which, according to the man page, can be
a boolean, or the special value "always". The man page states
that "true" is the default.

The default value is implemented in src/network/networkd-network.c
by setting field ipv6_accept_ra_start_dhcp6_client of network to
true. However, this field is not a boolean, but an enum type
IPv6AcceptRAStartDHCP6Client (src/network/networkd-ndisc.h).

Setting ipv6_accept_ra_start_dhcp6_client to true effectively
corresponds to the enum value IPV6_ACCEPT_RA_START_DHCP6_CLIENT_ALWAYS,
resulting in the DHCPv6Client setting having the default value
"always".

This patch changes the initialisation to the correct enum value
IPV6_ACCEPT_RA_START_DHCP6_CLIENT_YES.