[Rd] parallel and tempdir()

[Jeroen Ooms]

A forked process in mcparallel will wipe the (shared) tempdir when it
quits. I think this is almost always undesirable:

  file.exists(tempdir())
  parallel::mcparallel(q("no"))
  file.exists(tempdir()) # false
  file.create(tempfile()) # error

A simple solution might be to skip over R_CleanTempDir when
R_isForkedChild. Example patch: https://git.io/vDwyf

In addition, I would like to suggest to add an option to mcparallel to
set a custom tempdir in the fork to prevent potential conflicts. I
tried implementing this myself but found that setting R_TempDir is not
allowed by CMD check, and Sys_TempDir (which is what actually gets
wiped) is invisible behind attribute_hidden. So I suppose this may
only be done in base.