work around strange R 3.5.1 encoding issue

Open source companion commit for rstudio/rstudio-pro#479

remove C++11-isms

Fix some issues with running terminal on 64-bit rsession (Windows)

Fixes rstudio#2817

This has been somewhat non-deterministic. I'm seeing cases where
Win32 `executablePath()` returns an empty string, leading to mistakes
in locating the winpty.dll (and presumably other problems).
Also problems running terminal with 64-bit rsession build in dev
environment; hadn't tried that before, always used 32-bit.

Found some mention online that using `_pgmptr` for getting at
full path of current executable can have issues if used early in
startup, but it is safe to use `GetModuleFileName` early. So switched
to that approach.

Added an error log if there is a failure to find winpty.dll.

Made the decision on using 32-bit or 64-bit winpty.dll a compile-time
thing instead of runtime, since it's already known by virtue of which
rsession.exe is being built.

Merge pull request rstudio#2793 from rstudio/feature/rmarkdown-rstudi…

…o-version

Pass RStudio version to rmarkdown child process

reattach S3 overrides on load rather than attach

reattach S3 overrides on load rather than attach

reattach S3 overrides on load rather than attach

reattach S3 overrides on load rather than attach

allow S3 overrides to survive package attachment (for R Notebooks)