Move buffer name completion to Lisp and add affixation function

Move buffer name completion to Lisp and add affixation function

* lisp/minibuffer.el (minibuffer-narrow-buffer-completions): Update
doc string.
(buffers-except-current-if-switching,internal-complete-buffer-except)
(internal-complete-buffer): Replace with...
(completion-buffer-name-table): ...this.  New function.
(completion-buffer-name-affixation): New function.

* src/minibuf.c (Fread_buffer): Use 'completion-buffer-name-table'
instead of...
(Finternal_complete_buffer): ...this.  Remove.
(syms_of_minibuf): Update.

* doc/emacs/mini.texi (Completion Options): Mention buffer name
annotations in the documentation of 'completions-detailed'.

* etc/NEWS: Announce.