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/diff.c

@@ -1206,7 +1206,7 @@ void ex_diffpatch(exarg_T *eap)
       || (os_chdir((char *)dirbuf) != 0)) {
     dirbuf[0] = NUL;
   } else {
-    char *tempdir = (char *)vim_gettempdir();
+    char *tempdir = vim_gettempdir();
     if (tempdir == NULL) {
       tempdir = "/tmp";
     }