Add a new command `restart-emacs'

Add a new command `restart-emacs'

* doc/lispref/os.texi (Killing Emacs): Document it.

* lisp/files.el (save-buffers-kill-emacs): Add new RESTART parameter.
(restart-emacs): New function.

* src/emacs.c (terminate_due_to_signal, Fkill_emacs): Take an
optional RESTART parameter.

* test/lisp/files-tests.el
(files-tests-save-buffers-kill-emacs--confirm-kill-processes):
* src/xterm.c (x_connection_closed):
* src/xsmfns.c (Fhandle_save_session):
* src/keyboard.c (Fcommand_error_default_function, command_loop)
(command_loop_1, read_menu_command, read_event_from_main_queue)
(read_key_sequence, quit_throw_to_read_char):
* src/eval.c (process_quit_flag): Adjust Fkill_emacs callers.