client to listen on local sockets

[Antoine Martin]

Add local unix domain sockets and named pipes to #2406.

Those sockets will need a different identifier than the display.. not sure what. PID?

[Antoine Martin]

See also #2742, #2744

[Antoine Martin]

Mostly done in r27047.

Added more info in r27048.

Still TODO macos and win32 support: just socket paths?

[Antoine Martin]

Updates:

• r27049 name-pipe fix
• r27050 answer id requests so we can ignore client sockets in the mdns session gui
• r27051 win32 fix
• r27053 + r27055 macos fix
• r27054 macos: fix subcommands (ie: list)

Remaining issues:

• pids look like display numbers and could be interpreted as such if any of the client directories are mistaken for a server socket directory
• win32 shadow servers fail to create named-pipe listener (new? caused by r27049?)

[Antoine Martin]

Fixed:

• r27055 macos socket path confusion
• r27056 + r27058 + r27065: fix win32 shadow server named-pipes

---

Examples:

• list clients (to be improved in #2856):

  xpra list --socket-dirs=$XDG_RUNTIME_DIR/xpra/clients/
  

• if there's only one, this will work:

  xpra info --socket-dirs=$XDG_RUNTIME_DIR/xpra/clients/
  

• enable keyboard debug:

  xpra control socket:///$XDG_RUNTIME_DIR/xpra/clients/localhost.localdomain-131651 debug enable keyboard
  

etc..

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/2743