[ shell buffer ] How not to split the current window?

[Philippe Chauvat]

Hello

Not new to emacs but pretty old to lisp and other emacs programming
stuffs. I am running the following emacs version.

GNU Emacs 25.2.2 (x86_64-pc-linux-gnu, GTK+ Version 3.22.17)
 of 2017-08-01, modified by Debian
Copyright (C) 2017 Free Software Foundation, Inc.

since a previous update, when using M-x shell, the shell opens in a new
buffer, splitting the window. Therefore, using C-x b don't send me back
to the original buffer but to a third one. This is really not useful for
me and I would be glad that M-x shell just open a new buffer, as it
would do for other matters, without splitting the existing one.

I read the doc, probably not enough, I read the mailing list archive,
and did not find anything. Any help would be welcome.

Cheers
Philippe