[NEW] Support socat-style abstract AF_UNIX sockets via @ prefix

The tools already support UDS (AF_UNIX) sockets, for example via -s on the client tools, but abstract sockets (NUL-prefixed) cannot be expressed AFAIK, but are in many ways more convenient and desirable.

Suggestion: adopt the convention from socat/systemd/etc where @foo is interpreted as \0foo i.e. the abstract UDS foo

client

The client half of this is perhaps the more immediately useful, due to the use of proxy servers on app-boxes to allow short-lived processes to share long-lived connections to the server, with the proxy dealing with both Redis and TLS negotiation. This work is fairly simple #15572. Open question: upstream to hiredis first?

server

The work at the server is similar in scope and nature, but I have not yet explored aspects like replication, cluster, etc - from memory, this may fall naturally out of scope if replication is TCP-only. Arguably, the server is secondary as a server is often a shared resource, thus accessed over TCP between boxes - however, app-local on-box servers are not uncommon, so this may still be highly desirable, #15575