Indicate non-OK status of (currently) 10 CRAN mirrors.

git-svn-id: https://svn.r-project.org/R/trunk@67202 00db46b3-68df-0310-9c12-caf00c1e9a41
Add check_CRAN_mirrors().

git-svn-id: https://svn.r-project.org/R/trunk@67203 00db46b3-68df-0310-9c12-caf00c1e9a41