Make capabilities configurable in init() about fuser

[wmanley]

Regarding init: I've got some plans to use the typestate pattern to simplify this. The idea is that Init is then no longer a callback, but is a function instead. That way the filesystem callback no longer needs to contain state like is_initialised or do any synchronisation around that. So we'd have a type like:

pub struct ChannelNeedsInit{
    chan: Channel,
    init_msg: op::Init
};
impl ChannelNeedsInit {
    fn init(self, capabilities: u32, ...) -> Channel {
        ...
    }
}

Usage:

let (mount, chan_needs_init) = create_mount("/my/mountpoint");
if chan_needs_init.abi_version() > (7, 19) {
    // Blah blah
}
let chan = chan_needs_init.init(my_capabilities, abi_version, ...);
serve_sync(chan, handler);

I've not yet started implementation of this though. I guess it would serve some of the same purpose as Session, but it would not be parametrized on the Filesystem/Handler type, nor own the Mount.