check for both src/Makevars.in and src/Makevars

git-svn-id: https://svn.r-project.org/R/trunk@63399 00db46b3-68df-0310-9c12-caf00c1e9a41
Debugging improvements:  add s, f and help commands; clean up the browser behaviour following PR#14985.

git-svn-id: https://svn.r-project.org/R/trunk@63400 00db46b3-68df-0310-9c12-caf00c1e9a41