Add new args MESSAGE and TIMEOUT to set-transient-map (bug#21634)

Add new args MESSAGE and TIMEOUT to set-transient-map (bug#21634)

* lisp/subr.el (set-transient-map): Add new args MESSAGE and TIMEOUT.
(set-transient-map-timeout, set-transient-map-timer): New variables.

* lisp/international/emoji.el (emoji-zoom-increase):
* lisp/indent.el (indent-rigidly):
* lisp/face-remap.el (text-scale-adjust, global-text-scale-adjust):
Use the arg MESSAGE of set-transient-map.

* doc/lispref/keymaps.texi (Controlling Active Maps): Mention new args
MESSAGE and TIMEOUT of set-transient-map.