networkd: extend IPMasquerade to cover ipv6

Extend IPMasquerade to also cover ipv6.
For compatibility reasons with earlier releases IPMasquerade=yes
is identical to IPMasquerade=ipv4.

Use IPMasquerade=both to cover ipv6 as well as ipv4.

IPForward will now also enable ipv6 forwarding if IPMasquerade for ipv6 is enabled.