Make it possible to run a pure storage server (a node with no client)

[hacklschorsch]

In similar vein as #3814, I find it strange that Tahoe-LAFS seemingly can't run a storage server without also running a client.

This adds complexity, attack surface, and confusion: That the Tahoe client is a daemon with multiple listening sockets is confusing enough for a newbie, but that the server also is a client and one can't turn that off is really strange to someone who is used to the classic client/server paradigm.

Running tahoe --help emits, i.a., this item:

create-node Create a node that acts as a client, server or both.

I was able to create a client-only node, and a "both" node, but not a "server" node.

I propose to add an enabled config item for the client config section, similar to that of the other sections (storage, sftpd, helper, i2p, tor)