feat: stdpath('run'), /tmp/nvim.user/ #18993

Problem:
- Since c57f6b28d7 #8519, sockets are created in ~/.local/… but XDG
  spec says: "XDG_RUNTIME_DIR: Must be on the local filesystem", which
  implies that XDG_STATE_DIR is potentially non-local.
- Not easy to inspect Nvim-created temp files (for debugging etc).

Solution:
- Store sockets in stdpath('run') ($XDG_RUNTIME_DIR).
- Establish "/tmp/nvim.user/" as the tempdir root shared by all Nvims.
- Make ok() actually useful.
- Introduce assert_nolog().

closes #3517
closes #17093

src/nvim/msgpack_rpc/server.c

@@ -89,7 +89,7 @@ void server_teardown(void)
 ///
 /// Named pipe format:
 /// - Windows: "\\.\pipe\<name>.<pid>.<counter>"
-/// - Other: "~/.local/state/nvim/<name>.<pid>.<counter>"
+/// - Other: "/tmp/nvim.user/xxx/<name>.<pid>.<counter>"
 char *server_address_new(const char *name)
 {
   static uint32_t count = 0;
@@ -98,7 +98,7 @@ char *server_address_new(const char *name)
   int r = snprintf(fmt, sizeof(fmt), "\\\\.\\pipe\\%s.%" PRIu64 ".%" PRIu32,
                    name ? name : "nvim", os_get_pid(), count++);
 #else
-  char *dir = get_xdg_home(kXDGStateHome);
+  char *dir = stdpaths_get_xdg_var(kXDGRuntimeDir);
   int r = snprintf(fmt, sizeof(fmt), "%s/%s.%" PRIu64 ".%" PRIu32,
                    dir, name ? name : "nvim", os_get_pid(), count++);
   xfree(dir);