fuse: no longer require fuse to compile ipfs

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.

core/commands/internal/incfusever/incfusever.go

@@ -0,0 +1,13 @@
+// Package incfusever is only here to prevent go src tools (like godep)
+// from thinking fuseversion is not a required package. Though we do not
+// actually use github.com/jbenet/go-fuse-version as a library, we
+// _may_ need its binary. We avoid it as much as possible as compiling
+// it _requires_ fuse headers. Users must be able to install go-ipfs
+// without also installing fuse.
+package incfusever
+
+import (
+	fuseversion "github.com/jbenet/go-ipfs/Godeps/_workspace/src/github.com/jbenet/go-fuse-version"
+)
+
+var _ = fuseversion.LocalFuseSystems