[WIP] Sanitize get output paths for external file systems

[djdv]

This allows illegal paths in IPFS to be get-able on Windows, it also accounts for : on Darwin, other platforms need just implement their own sanitize function and alter the build flag if they have special requirements.

Darwin substitutes : with a hyphen - as this is what Finder does if you try to make a file with a colon.

TODO:

• See if there's a more optimal way to handle this
• ^ is allowed on Windows/NTFS but not Windows/FAT32, accounting for this requires detecting the output target filesystem.
• Tests
  Currently I'm manually using a hash generated via cmd (^ is the escape character there)

ipfs files rm -r /test
ipfs files mkdir -p "/test/badnames/:^>*^"bad^|name^"^\*^<:.   "
ipfs files cp /ipfs/QmakyK2Xktmn9HnVGuaDZGtjkPq3HCfmF7SPJVWvTYwduX "/test/badnames/:^>*^"bad^|name^"^\*^<:.   /Is this a file?  ."
ipfs files stat /test

it resolves to QmP6tsejWJczc348FwKncykrRDvbVdXtb3oCCknFMQUii6\badnames\%3A^%3E%2A^bad%7Cname%5C%2A%3C%3A\Is this a file%3F which is ugly but better than failure imo.

[djdv]

Closing in favour of whyrusleeping/tar-utils#2
I'll make a separate PR for testing get with this later in the go-ipfs repo.