Add support for unix sockets

[aaronlehmann]

For the sake of completeness, Continuity manifests should include unix sockets. However, it is not generally useful for continuity to create these when applying a manifest. Thus, apply and verify should ignore unix sockets by default. A special flag should allow them to be applied and verified.

[AkihiroSuda]

I think this is already covered (os.ModeSocket)

https://github.com/stevvooe/continuity/blob/master/proto/manifest.proto

    // Mode defines the file mode and permissions. We've used the same
    // bit-packing from Go's os package,
    // http://golang.org/pkg/os/#FileMode, since they've done the work of
    // creating a cross-platform layout.
    uint32 mode = 6;