mount: support 9P2000.L as a client and server

[djdv]

Our old implementation already supported hosting a 9P server of any of our FileSystem interface (this predated Go's fs.FS).
Our new fs daemon code itself utilizes 9P and has lots of functionality for managing sockets, servers, etc.

We should be able to both mount and serve 9P through any of the combinations.
E.g.
Serving local IPFS node over a TCP socket: fs mount 9p ipfs /ip4/127.0.0.1/tcp/564
Mounting that server via a 9P client mapped to the FUSE API: fs mount fuse 9p /ip4/127.0.0.1/tcp/564 /mnt/9