Add `remember-notes' function to store random notes across Emacs

Add `remember-notes' function to store random notes across Emacs
restarts.
* remember.el (remember-data-file): Add :set callback to affect
notes buffer (if any).
(remember-notes): New command.
(remember-notes-buffer-name, bury-remember-notes-on-kill):
New defcustoms for the `remember-notes' function.
(remember-notes-save-and-bury-buffer): New command.
(remember-notes-mode-map): New variable.
(remember-mode): New minor mode.
(remember-notes--kill-buffer-query): New function.
* lisp/startup.el (initial-buffer-choice): Add notes to custom type.
* src/buffer.c (FKill_buffer): Run `kill-buffer-query-functions'
before checking whether buffer is modified.  This lets
`kill-buffer-query-functions' cancel killing of the buffer or save
its content before `kill-buffer' asks user the "Buffer %s
modified; kill anyway?" question.

* remember.el (remember-append-to-file):
Don't mix `find-buffer-visiting' and `get-file-buffer'.

* lisp/files.el (find-file-noselect): Simplify conditional expression.