Log Message:
Add the CRAN archives to R package MASTER_SITES.

Distfiles for older R packges are moved on CRAN from contrib/ to
contrib/Archive/${R_PKGNAME}/.  As a result, they may be inaccessible to
'make fetch'.  Add the additional directory to MASTER_SITES in
math/R/Makefile.extension so that older distfiles will be retrieved
directly from CRAN.  In addition, remove the definition of MASTER_SITES
from math/R-prob/Makefile, which is the only package directly referring to
the distfile archive.  At this point all R packages should rely on
math/R/Makefile.extension to define MASTER_SITES; they should not be
defining MASTER_SITES directly.