[Rd] change R_LIBS_USER default location

[Matsievskiy S.V.]

Dear r-devel team,

By default, R_LIBS_USER is set to $HOME/R.
I think that it would be better to conform with XDG Base Directory
Specification (
https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html
) and use $XDG_DATA_HOME/R (~/.local/share/R) instead.

P.S. This message was originally sent to Debian bug report system (
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=928653). Debian R
maintainer Dirk Eddelbuettel recommended sending this issue to r-devel.

--
Best regards,
S.V. Matsievskiy