bug#14147: 24.3.50; turn on `minibuffer-depth-indicator-mode' by default

["Drew Adams" <drew.adams@oracle.com>]

It is unfortunate that this minor mode is still not turned on by
default.  There is no reason not to do so.  The mode has no effect when
there is no recursive minibuffer, which is most of the time.  And when
there is a recursive minibuffer it is essential that that fact be
communicated to users, one way or another.  This mode provides one way.
 
mb-depth.el has been present in the code for years now, but it is still
essentially ignored, unknown to most users.  Please put it to use,
letting users know when they are in a recursive minibuffer, by default.
Novice users are the least likely to discover this feature, and they are
perhaps the most in need of its aid.
 
This is no different from Emacs automatically adding nested `[...]' to
the mode line to indicate recursive editing levels.  No one would
suggest that that indicator be turned off by default and that users be
forced to learn about it on their own and turn it on.  The same should
be the case for indicating recursive minibuffer levels.  Should be a
no-brainer.

In GNU Emacs 24.3.50.1 (i386-mingw-nt5.1.2600)
 of 2013-03-28 on ODIEONE
Bzr revision: 112173 monnier@iro.umontreal.ca-20130328163306-419khox8aneaoaxv
Windowing system distributor `Microsoft Corp.', version 5.1.2600
Configured using:
 `configure --with-gcc (4.7) --no-opt --enable-checking --cflags
 -IC:/Devel/emacs/build/include --ldflags -LC:/Devel/emacs/build/lib'