IPFS permits undesirable paths

IPFS path components (that is, IPFS merkledag node link names) can contain undesirable things:
- C string terminators (`0x00`, ASCII `NUL`)
- UNIX newlines (`0x0A`, `"\n"`, ASCII `LF`)
- Tabs (`0x09`, `"\t"`)
- Escape sequences (`0x1B`, ASCII `ESC`)
- Every other control character (`0x00`…`0x1F`, `0x7F`)
- Slashes (`0x2F`, `"/"`)
- Bytes which are illegal in UTF-8 (`0xFE`, `0xFF`)
- Byte sequences which are invalid UTF-8 (malformed sequences, like `0x80` in isolation)
- UTF-8 sequences that encode invalid code points (i.e. `U+D800`…`U+D8FF`)
- UTF-8 sequences that are overlong (i.e. `0xC0 80`, `0xE0 80 80`, `F0 80 80 80` all decode as `U+0000`, which is `NUL`)

Path components can also be strings that are commonly understood as path components with a special meaning, namely `"."` and `".."`.

I propose that path components (link names) be restricted both by specification and implementation, preferably by defining "valid" path components in a way that excludes the above. `go-ipfs` should refuse to create invalid links. `go-ipfs` should also refuse to process any invalid links it sees, either by discarding the link or discarding the node, and that behavior should be specified as well.