makeClusterPSOCK(): Automatically pass down current .libPaths() to localhost cluster nodes?

[HenrikBengtsson]

From https://github.com/HenrikBengtsson/future/issues/435:

Should localhost PSOCK cluster node inherit .libPaths() from parent R session?

This could be done by introducing a new default rscript_libs = "<auto>" and have that to decide what the default rscript_libs should become. This can then be disabled by explicitly specifying rscript_libs = NULL.