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⚠ this commit makes your current configs unusable, as the
default bootstrap peers. You may need to edit your config.

Go from:

```js
Bootstrap: [
  {
    "Address": "/ip4/104.131.131.82/tcp/4001",
    "PeerID": "QmaCpDMGvV2BGHeYERUEnRQAwe3N8SzbUtfsmvsqQLuvuJ"
  }
]
```

To:
```js
Bootstrap: [
  "/ip4/104.131.131.82/tcp/4001/ipfs/QmaCpDMGvV2BGHeYERUEnRQAwe3N8SzbUtfsmvsqQLuvuJ"
]
```

Addresses bug #689

This commit removes the dependency on go-fuse-version, and thus the
fuse headers. It also introduces an elaborate troubleshooting process
that diagnoses whether fuse installed -- and which version -- with as
little requirements as possible (attept to use sysctl, fall-back on
the go-fuse-version binary, etc). It then nicely instructs the user
what to do next.

squash! core/commands: Save 'get' output to disk

core/commands: get: Fixed PostRun signature

corerepo package name provides sufficient context (even if renamed by
caller)

@jbenet @whyrusleeping

was the 1<<16 intentional? replaced the raw methods with wrappers.