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

[Eli Zaretskii]

> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Tue, 14 Apr 2015 05:37:18 +0300
> 
> Is there a reason it's t by default?

History and custom.

> 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.

Each major mode can (and some already do) have its own default.

> 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.

If the customizations are due to language guidelines/culture, then
setting this to nil in the major mode is TRT.