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/os/fs.c

@@ -126,7 +126,7 @@ bool os_isrealdir(const char *name)
   }
 }
 
-/// Check if the given path is a directory or not.
+/// Check if the given path exists and is a directory.
 ///
 /// @return `true` if `name` is a directory.
 bool os_isdir(const char_u *name)
@@ -791,6 +791,27 @@ int os_setperm(const char *const name, int perm)
   return (r == kLibuvSuccess ? OK : FAIL);
 }
 
+#ifdef UNIX
+/// Checks if the current user owns a file.
+///
+/// Uses both uv_fs_stat() and uv_fs_lstat() via os_fileinfo() and
+/// os_fileinfo_link() respectively for extra security.
+bool os_file_owned(const char *fname)
+  FUNC_ATTR_NONNULL_ALL
+{
+  uid_t uid = getuid();
+  FileInfo finfo;
+  bool file_owned = os_fileinfo(fname, &finfo) && finfo.stat.st_uid == uid;
+  bool link_owned = os_fileinfo_link(fname, &finfo) && finfo.stat.st_uid == uid;
+  return file_owned && link_owned;
+}
+#else
+bool os_file_owned(const char *fname)
+{
+  return true;  // TODO(justinmk): Windows. #8244
+}
+#endif
+
 /// Changes the owner and group of a file, like chown(2).
 ///
 /// @return 0 on success, or libuv error code on failure.