bug#16571: 24.3.50; Add :group to `defcustom's in minibuffer.el

[Drew Adams]

Subject line says it all.  None of the defcustoms in `minibuffer.el'
have a :group.  They all should.

Zero :group entries is not advised.  Adding a :group helps users find
the option.

Presumably, at least group `minibuffer' should apply to the options
defined in `minibuffer.el', which defines that group.  Other groups
might also apply.


In GNU Emacs 24.3.50.1 (i686-pc-mingw32)
 of 2014-01-23 on ODIEONE
Bzr revision: 116129 monnier@iro.umontreal.ca-20140123150141-qopqqhpm8jqo8a18
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/Devel/emacs/binary --enable-checking=yes,glyphs
 'CFLAGS=-O0 -g3' LDFLAGS=-Lc:/Devel/emacs/lib
 CPPFLAGS=-Ic:/Devel/emacs/include'