[Rd] Tabs in R source code

[Peng Yu]

Some lines are indented by tabs and some lines are indented by spaces,
in R source code. This might due to the fact that the source code is
not from a single person. But I think that it is better to enforce a
single convention.

My editor defaults a tab to 8 spaces. So some source doesn't seem to
be indented correctly in my editor. Since tab may be displayed
differently in different editor, I recommend to replace all the tabs
by the appropriate number of spaces to ensure the same indentation
display.

Could somebody update the source code in the future version by
replacing tabs by spaces and enforce such a coding convention in the
future?