Be stricter about IPFS parent directory path traversal.

#### Version information:
> go-ipfs version: 0.4.11-dev-48476b292
Repo version: 5
System version: amd64/linux
Golang version: go1.8.3

#### Type:

Bug

#### Severity:

Low

#### Description:

Currently, we all walking up to a parent directory using `..` for convenience. However, we allow this behavior in two cases where it doesn't really make sense:

1. After a terminal file. That is, `/ipfs/QmS4ustL54uo8FzR9455qaxZwuMiUhyvMcX9Ba8nUH4uVv/readme/../readme` works. Unfortunately, fixing this without simply removing parent directory traversal may be a bit difficult.
2. At the root. That is, `/ipfs/QmT78zSuBmuS4z925WZfrqQ1qHaJ56DQaTfyMUF7F8ff5o/../QmS4ustL54uo8FzR9455qaxZwuMiUhyvMcX9Ba8nUH4uVv/readme`. Actually, we allow any number of `..` at the root: `/ipfs/QmT78zSuBmuS4z925WZfrqQ1qHaJ56DQaTfyMUF7F8ff5o/../../../adsf/../../../QmS4ustL54uo8FzR9455qaxZwuMiUhyvMcX9Ba8nUH4uVv/readme'.

I'm bringing this up because it can make security hard to reason about. That is, if someone naively parses `/ipfs/QmGood/../QmBad/badCode` and verifies that `QmGood` is trusted, they'll fetch `badCode` without realizing it. It was only dumb luck that prevented this from being an issue for the gateway @kyledrake is working on (a gateway with proper security origins).

Start by locating the IPFS path-resolution entry point that handles `..`, then reproduce the terminal-file and root traversal examples from the issue. Add regression coverage for both cases and verify that traversal cannot escape the intended IPFS root or continue through a terminal file.

Written by the indexing model from the issue text.