[FEATURE] mfsbdev as standard (TCP/Unix Socket) NBD server

[tianon]

Related to #527 (comment) and #113, I finally looked deeper at mfsbdev again, especially as I found https://manpages.debian.org/bookworm/qemu-system-x86/qemu-system-x86_64.1.en.html#NBD which makes it ~easy for QEMU to connect directly to an NBD server over TCP or Unix Domain Sockets, which seemed perfect!

However, I quickly re-realized that mfsbdev seems to be more of a helper for directly creating block devices on a host that are backed by MooseFS (via NBD), and I could not find a way to expose a "standard" NBD server from it.

So, in short, my feature request is that mfsbdev have some way to just expose a "proper" NBD server over TCP and/or unix socket instead of just the (AFAICT) custom protocol it currently supports. 🙇 ❤️

[tianon]

A direct iSCSI server would also be interesting, especially as it could then be more easily used for things like booting a machine off the network with storage on MooseFS, but I think the iSCSI protocol is a lot more complicated than NBD (so that'd be a whole separate feature request / larger engineering effort, I think). 😄