NAT_ENABLED error reported when set to false

[unquietwiki]

Setting both SUBSPACE_IPV6_NAT_ENABLED and SUBSPACE_IPV4_NAT_ENABLED to 0 or false will cause errors saying you have to have at least one NAT turned on. That behavior only makes sense if SUBSPACE_ALLOWED_IPS is not default.

First discovered in #203

[riconnon]

@unquietwiki
I'm trying to follow your statement:

That behavior only makes sense if SUBSPACE_ALLOWED_IPS is not default

I'm not sure why this is related to SUBSPACE_ALLOWED_IPS at all? Simply, it seems to be that it is always a valid configuration to not NAT if one doesn't want to and that this check could simply be removed.

[riconnon]

It looks like SUBSPACE_DISABLE_MASQUERADE might be what's intended to be used here, but notably that also disables creating the firewall rules to allow forwarding.
I think that SUBSPACE_IPV4_NAT_ENABLED and SUBSPACE_IPV6_NAT_ENABLED might be poorly named and should not include the word NAT since they also gate configuring the server interface.