bug#12053: 24.1.50; `nobreak-char-display' should be a defcustom

[Drew Adams]

Seems obvious from the doc string that this should be a user option.  It
is users who should decide how to display these characters.

In GNU Emacs 24.1.50.1 (i386-mingw-nt5.1.2600)
 of 2012-07-22 on MARVIN
Bzr revision: 109189
vincentb1@users.sourceforge.net-20120722211412-mnq2ih82me0bpuhx
Windowing system distributor `Microsoft Corp.', version 5.1.2600
Configured using:
 `configure --with-gcc (4.6) --no-opt --enable-checking --cflags
 -ID:/devel/emacs/libs/libXpm-3.5.8/include
 -ID:/devel/emacs/libs/libXpm-3.5.8/src
 -ID:/devel/emacs/libs/libpng-dev_1.4.3-1/include
 -ID:/devel/emacs/libs/zlib-dev_1.2.5-2/include
 -ID:/devel/emacs/libs/giflib-4.1.4-1/include
 -ID:/devel/emacs/libs/jpeg-6b-4/include
 -ID:/devel/emacs/libs/tiff-3.8.2-1/include
 -ID:/devel/emacs/libs/gnutls-3.0.9/include
 -ID:/devel/emacs/libs/libiconv-1.13.1-1-dev/include
 -ID:/devel/emacs/libs/libxml2-2.7.8/include/libxml2'