networkd: bump MTU to 1280 for interfaces which have IPv6 enabled (#3077)

IPv6 protocol requires a minimum MTU of 1280 bytes on the interface.
This fixes #3046.

Introduce helper link_ipv6_enabled() to figure out whether IPV6 is enabled.
Introduce network_has_static_ipv6_addresses() to find out if any static
ipv6 address configured.
If IPv6 is not configured on any interface that is SLAAC, DHCPv6 and static
IPv6 addresses not configured, then IPv6 will be automatically disabled for that
interface, that is we write "1" to /proc/sys/net/ipv6/conf//disable_ipv6.