bug#20322: 25.0.50; indent-tabs-mode should default to nil

[Dmitry Gutov]

Is there a reason it's t by default?

There are different personal preferences for indentation, but spaces are
used quite a bit more often than tabs, in most languages I'm familiar with.

Having it be nil by default (but set locally by modes that really need
it, like makefile-mode) will mean that most users won't have to
customize it.

In GNU Emacs 25.0.50.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.12.2)
 of 2015-04-14 on axl
Repository revision: 56dfd3de9c63dbb6325129de8e38c420808146c9
Windowing system distributor `The X.Org Foundation', version 11.0.11601901
System Description:     Ubuntu 14.10