bug#902: select-active-regions only half-working

[Stefan Monnier]

> +        (save-excursion
> +          (set-buffer value)

Aka "(with-current-buffer value".
Again, since this code is present at several places, move it into its
own function.  Basically this function could turn the buffer value into
a "cons of markers" value, so you can use it everywhere and reuse the
code that handles a cons of markers.

Also the select-active-regions docstring shouldn't recommend to turn on
transient-mark-mode since it's already ON by default.


        Stefan