bug#18693: 25.0.50; backup user options

[Drew Adams]

There are several user options for customizing file backups
(`backup-by-*').  Wading through the doc strings or Customize buffers
for them, it seems that they could all be consolidated under option
`make-backup-files', by letting different non-nil values provide the
different backup behaviors.

To be clear: I really don't care about this.  Consider it feedback that
it seems that Emacs is making things unnecessarily complicated for
users, here, and it should be possible to do things better.  If no one
wants to do that, fine.

Also, things have been like this since at least Emacs 20, and users
already have various `backup-by-*' options in their custom or init
files.  So if this were simplified then some remapping/aliasing would
need to happen.

In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2014-10-06 on LEG570
Bzr revision: 118063 monnier@iro.umontreal.ca-20141006174756-y7ha091r491l1ijw
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking=yes,glyphs 'CFLAGS=-O0 -g3'
 CPPFLAGS=-DGLYPH_DEBUG=1'