Add a DirectoryMode to the Emacs Server example

* doc/emacs/misc.texi (Emacs Server): Update example

* doc/emacs/misc.texi (Emacs Server): The socket containing directory
is per default created with permissions 0755 by the socket-unit.
However this is considered unsafe since commit [1], so enhance unit
example with systemd configuration directive `DirectoryMode=' to
create it with safe permissions, see
https://www.freedesktop.org/software/systemd/man/systemd.socket.html#DirectoryMode=

[1] 2003-04-12 "(server-socket-name): Use new safe location for socket."

Copyright-paperwork-exempt: yes

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