Tstatfs type handling is a bit strange

[garlick]

Problem: when responding to Tstatfs, diod returns V9FS_TYPE unless the user has explicitly configured statfs_passthru = 1, but in diod's case, the default should probably be passthru.

Maybe it makes more sense to use V9FS_TYPE for synthetic file systems (e.g. the built-in ctl) and the remote file system's type for all other cases.