Implement fuse filesystem mount

[stevvooe]

It would be nice to be able to mount a continuity manifest via fuse via command:

continuity mount --fuse <mount> <manifest> <root>

This would only serve files identified by the manifest and only if their hash matched. We use --fuse to allow other kinds of mount implementations (imagine http, tar, etc.). Such a command should take the same options required to map uids, links, etc (not defined yet).

[jlhawn]

This should enable records for special files too:

• block/char devices
• unix sockets
• pipes (FIFOs)
• etc.

Your FUSE filesystem need not implement them, only report that they exist via dir entries and node attrs, the kernel will handle dispatching device requests to the proper driver (though major/minor mappings will likely be different between systems) and the kernel will also handle things like sockets and pipes. Gotta use the dev mount option though.

Discussed earlier IRL; Wasn't sure if I should open another issue since it's related to this one.

[stevvooe]

@jlhawn This should probably be another issue. The support for devs and sockets is shaky at this point.