don't scan for .libPaths() changes while debugging
don't Step Into functions run by RStudio itself
protect additional vulnerable executions with DisableStepInto
don't restore browser command after debugging ends
use RDEBUG flag on global environment instead of R_BrowserLastCommand
save/restore debug flag on non-global environments
Merge pull request #163 from rstudio/bugfix/toplevel-stepinto

Fix runaway recursion resulting from Step Into