server-start fails to start, with the message:

apply: Cannot bind server socket: No such file or directory

To reproduce:

1. Kill any existing emacs-server
2. Rename or delete $XDG_RUNTIME_DIR/emacs
3. Execute server-start


The problem is that the directory specified in XDG_RUNTIME_DIR existed, but
with no emacs subdirectory. I do believe that emacs ought simply create the
directory