Merge bitcoin/bitcoin#27679: ZMQ: Support UNIX domain sockets

21d0e6c7b7 doc: release notes for PR 27679 (Matthew Zipkin)
791dea204e test: cover unix sockets in zmq interface (Matthew Zipkin)
c87b0a0ff4 zmq: accept unix domain socket address for notifier (Matthew Zipkin)

Pull request description:

  This is a follow-up to https://github.com/bitcoin/bitcoin/pull/27375, allowing ZMQ notifications to be published to a UNIX domain socket.

  Fortunately, libzmq handles unix sockets already, all we really have to do to support it is allow the format in the actual option.

  [libzmq](https://libzmq.readthedocs.io/en/latest/zmq_ipc.html) uses the prefix `ipc://` as opposed to `unix:` which is [used by Tor](https://gitlab.torproject.org/tpo/core/tor/-/blob/main/doc/man/tor.1.txt?ref_type=heads#L1475) and now also by [bitcoind](a85e5a7c9a/doc/release-notes-27375.md (L5)) so we need to switch that internally.

  As far as I can tell, [LND](d20a764486/zmq.go (L38)) supports `ipc://` and `unix://` (notice the double slashes).

  With this patch, LND can connect to bitcoind using unix sockets:

  Example:

  *bitcoin.conf*:
  ```
  zmqpubrawblock=unix:/tmp/zmqsb
  zmqpubrawtx=unix:/tmp/zmqst
  ```

  *lnd.conf*:
  ```
  bitcoind.zmqpubrawblock=ipc:///tmp/zmqsb
  bitcoind.zmqpubrawtx=ipc:///tmp/zmqst
  ```

ACKs for top commit:
  laanwj:
    Code review ACK 21d0e6c7b7
  tdb3:
    crACK for 21d0e6c7b7.  Changes lgtm. Will follow up with some testing within the next few days as time allows.
  achow101:
    ACK 21d0e6c7b7
  guggero:
    Tested and code review ACK 21d0e6c7b7

Tree-SHA512: ffd50222e80dd029d903e5ddde37b83f72dfec1856a3f7ce49da3b54a45de8daaf80eea1629a30f58559f4b8ded0b29809548c0638cd1c2811b2736ad8b73030

doc/release-notes-27679.md

@@ -0,0 +1,2 @@
+- unix socket paths are now accepted for `-zmqpubrawblock` and `-zmqpubrawtx` with
+the format `-zmqpubrawtx=unix:/path/to/file`