message uses minibuffer-message in the active minibuffer (bug#17272 bug#19064)

message uses minibuffer-message in the active minibuffer (bug#17272 bug#19064)

* doc/lispref/display.texi (Displaying Messages): Explain the
behavior of using minibuffer-message if the minibuffer is active.

* src/editfns.c (Fmessage_in_echo_area): New function with body
copied from Fmessage.
(Fmessage): Call minibuffer-message in the active minibuffer,
otherwise call Fmessage_in_echo_area.
(message-in-echo-area): New variable.

* lisp/isearch.el (isearch--momentary-message, isearch-message):
* lisp/minibuffer.el (minibuffer-message, minibuffer-completion-help):
Use 'message-in-echo-area' instead of 'message' where necessary.

* lisp/autorevert.el (auto-revert-handler):
* lisp/man.el (Man-bgproc-sentinel):
* lisp/subr.el (do-after-load-evaluation):
Revert recent changes that replaced 'message' with 'minibuffer-message'.
This is not needed anymore since 'message' uses 'minibuffer-message'
in the active minibuffer.