Fix incompleteness in the implementation of minibuffer-follows-selected-frame
In particular, add a new value to the variable, and fix several bugs apparent with the implementation up till now. * doc/emacs/mini.texi (Basic Minibuffer): Add a description of the new non-nil, non-t value of minibuffer-follows-selected-frame. * doc/emacs/trouble.texi (Quitting): Add a description of how C-g handles recursive minibuffers when typed in one which isn't the most nested. * doc/lispref/minibuf.texi (Intro to Minibuffers): Add an @dfn for "active minibuffer". (Minibuffer Commands): Document that exit-minibuffer throws an error when not invoked from the innermost Minibuffer. (Recursive Mini): Amend the description of the visibility of outer level minibuffers. (Minibuffer Misc): In the description of the minibuffer hooks, replace "the minibuffer" with "a minibuffer". * etc/NEWS (Entry announcing minibuffer-follows-selected-frame): Add a description of the new non-nil, non-t value. * lisp/cus-start.el (top level): make the customize entry for minibuffer-follows-selected-frame a choice between three entries. * lisp/minibuffer.el (exit-minibuffer): throw an error when we're not in the most nested minibuffer. (top level): Bind C-g to abort-minibuffers in minibuffer-local-map. * lisp/window.el (window-deletable-p): return the symbol `frame' when (amongst other things) minibuffer-follows-selected-frame is t. * src/eval.c (internal_catch): Add a mechanism to (throw 'exit t) repeatedly when the throw currently being processed doesn't terminate the current minibuffer. * src/lisp.h (this_minibuffer_depth): New extern declaration (minibuf_level): extern declaration moved here from window.h. * src/minibuf.c (minibuffer_follows_frame, minibuf_stays_put) (minibuf_moves_frame_when_opened): New and amended functions to query the value of minibuffer-follows-selected-frame. (choose_minibuf_frame): check (minibuf > 1) in place of (minibufer > 0) at a particular place. At another place, check that an alleged frame is so and is live. Before selecting a non-miniwindow on a different frame, ensure it really is a different frame. (move_minibuffer_onto_frame): Stack up all recursive minibuffers on the target frame. Check the minibuf_window isn't in the old frame before setting that frame's miniwindow to an inactive minibuffer. (Finnermost_minibuffer_p, Fabort_minibuffers): New primitives. (this_minibuffer_depth): New function. (read_minibuf): Record the calling frame in a variable, and switch back to it after the recursive edit has terminated normally, using select-frame-set-input-focus. Stack up all the recursive minibuffers on the miniwindow where a new minibuffer is being opened. After the recursive edit, switch the selected window away from the expired minibuffer's window. (nth_minibuffer): New function. (minibuffer-follows-selected-frame): Change from a DEFVAR_BOOL to a DEFVAR_LISP. * src/window.c (decode_next_window_args): Set *minibuf to w's mini-window's content when that content is a minibuffer. * src/window.h (minibuf_level) Declaration moved from here to lisp.h.