[Rd] simplifying number of R installations on disk

[Gabor Grothendieck]

If R-2.10.0 were the current version of R then by default there will
be an R-2.10.0 directory on your machine and when R-2.10.1 comes along
a new directory R-2-10.1 will be created if you use the installation
defaults.

I normally don't use the defaults but rather put all R-2.10.* versions
in the same directory so that R-2.10.1 overwrites R-2.10.0; however,
if the first or second number in the version change then I do use a
new directory.  Also I put any alpha or beta versions of R-2.10 in
R-2.10 again overwriting any prior R-2.10 version.

Actually this is already how it works for win-library.  That is, R
automatically generates a win-library\2.9, a win-library\2.10 but
there will not be automatically generated separate libraries for the
third level version number or for alpha or beta versions.

I think it would make more sense for the installation procedure of R
itself to use the same scheme that win-library uses since otherwise
there is an accumulation of too many directories as R changes.  It
should still be possible to have the current scheme for those who wish
to retain it but I doubt that most people really want to keep separate
third level versions separately so it would not be the default;
instead, a scheme that produces R-2.10, R-2.11, etc. R directories
would be the default.