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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.

Exposing the NAT to the core is unnecessary. The Host can take
care of it. If a need emerges, we can address it then.

- allow retrieving addresses
- allow notifications on mapping changes
- allow lifecycle mgmt (future commit will manage it)

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.