Define /unix (#174)

Adds a protocol note for how to encode paths to Unix domain sockets
as strings, that may include the delimiting character of `/`.

This allows us to append other tuples to the multiaddr while also
ensuring we can round-trip the address to a string and back.

This doesn't affect the binary representation of the multiaddr since
everything is length-delimited.

Takes inspiration from #164 and proposes using percent encoding for the
segment, the same as the `/http-path` component.

Author: achingbrain

protocols.csv

@@ -13,7 +13,7 @@ code,	size,	name,	comment
 132,	16,	sctp,
 301,	0,	udt,
 302,	0,	utp,
-400,	V,	unix,
+400,	V,	unix,	Percent-encoded path to a Unix domain socket
 421,	V,	p2p,	preferred over /ipfs
 421,	V,	ipfs,	backwards compatibility; equivalent to /p2p
 444,	96,	onion,
@@ -39,4 +39,4 @@ code,	size,	name,	comment
 280,	0,	webrtc-direct, ICE-lite webrtc transport with SDP munging during connection establishment and without use of a STUN server
 281, 	0,	webrtc, webrtc transport where connection establishment is according to w3c spec
 290,	0,	p2p-circuit,
-777,	V, memory, in memory transport for self-dialing and testing; arbitrary 
+777,	V, memory, in memory transport for self-dialing and testing; arbitrary

protocols/unix.md

@@ -0,0 +1,37 @@
+# `unix`
+
+This protocol encodes a Unix domain socket path to a resource. In the string
+representation, the path is encoded in a way consistent with a single URI Path
+segment per [RFC 3986 Section 3.3](https://datatracker.ietf.org/doc/html/rfc3986#autoid-23).
+
+Specifically following the grammar of a single `segment-nz`. In the binary
+representation, no encoding is needed as the value is length prefixed.
+
+When comparing multiaddrs, implementations should compare their binary
+representation to avoid ambiguities over which characters were escaped.
+
+## Examples
+
+The following is a table of examples converting some common Unix paths to their
+Multiaddr string form.
+
+| Unix Path                   | Multiaddr string form                   |
+| --------------------------- | --------------------------------------- |
+| /                           | `/unix/%2F`                             |
+| /file.socket                | `/unix/%2Ffile.socket`                  |
+| /dir/file.socket            | `/unix/%2Fdir%2Ffile.socket`            |
+| /dir/file.socket/p2p/12D... | `/unix/%2Fdir%2Ffile.socket/p2p/12D...` |
+| /tmp/foo/../bar             | `/unix/%2Ftmp%2Ffoo%2F..%2Fbar`         |
+| /%2F                        | `/unix/%252F`                           |
+| /a%20space                  | `/unix/%2Fa%2520space`                  |
+| /a%2Fslash                  | `/unix/%2Fa%252Fslash`                  |
+| socket                      | `/unix/socket`                          |
+
+## Usage
+
+`/unix` would typically be found at the start of a multiaddr, however it may
+appear anywhere, for example in the case where we route through some sort of
+proxy server or SSH tunnel.
+
+The absence of a `/` character at the start of the decoded address indicates a
+relative path, otherwise the path is absolute.