Make 'server-stop-automatically' into a defcustom

Make 'server-stop-automatically' into a defcustom

This changes the meaning of the (formerly internal) variable
'server-stop-automatically': it now always holds the requested
configuration, even when Emacs was not started as a daemon
(bug#59668).

* lisp/server.el (server-stop-automatically): Convert the variable to
a defcustom, and make the function simply set the defcustom.
(server-stop-automatically--timer): New variable.
(server-apply-stop-automatically): New function...
(server-stop, server-start): ... call it.
(server-save-buffers-kill-terminal): Adjust the conditions for
stopping automatically to account for the change of meaning for
'server-stop-automatically'.
(server-stop-automatically--handle-delete-frame): Remove unnecessary
test of the 'server-stop-automatically' option; this hook is only set
when it should do its job.
(server-stop-automatically--maybe-kill-emacs): Update docstring.

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