WISH: Add option for specifying architecture in makeClusterFunctionsLocal()

[HenrikBengtsson]

On (at least) Windows, the R installation comes with both 32-bit and 64-bit binaries, i.e. architectures i386 and x64. The makeClusterFunctionsLocal() function uses the binary of the currently running R session;

file.path(R.home("bin"), "Rscript")

However, there might be circumstances where you wish to run the job using a specific architecture. For instance, http://stackoverflow.com/questions/13070706/how-to-connect-r-with-access-database-in-64-bit-window.

Thus, it could be useful to support:

makeClusterFunctionsLocal(arch="i386")

where makeClusterFunctionsLocal() looks something like:

## Validate during setup
if (is.null(arch)) {
  bin <- file.path(R.home("bin"), "Rscript")
} else {
  path <- file.path(R.home(), "bin", arch)
  if (!file_test("-d", path)) stop("Non-supported architecture: ", arch)
  bin <- file.path(path, "Rscript")
}

submitJob = function(conf, reg, job.name, rscript, log.file, job.dir, resources, arrayjobs) {
   system2(bin, ...)
}

This might apply to other backends as well.