With minibuffer-follows-selected-frame `hybrid', preserve recursive Mbuffers

With minibuffer-follows-selected-frame `hybrid', preserve recursive Mbuffers

...when enable-recursive-minibuffers is non-nil, and several minibuffers are
activated from different frames.  Also set the major mode of a reused active
minibuffer to `fundamental-mode' - up till now it's been
minibuffer-inactive-mode.

* src/minibuf.c (read_minibuf): with the indicated settings of variables,
"stack up" all containing minibuffers on the mini-window of the current
frame.  Delete another, now superfluous such stacking up.
(set_minibuffer_mode): New function.
(get_minibuffer): Call the above new function (twice), in place of inline
code, ensuring active minibuffers are never left in minibuffer-inactive-mode.