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

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 = ""` and have that to decide what the default `rscript_libs` should become. This can then be disabled by explicitly specifying `rscript_libs = NULL`.

Start at makeClusterPSOCK() and trace how rscript_libs is interpreted for localhost PSOCK nodes. Implement the proposed default behavior so nodes inherit the parent .libPaths(), while explicitly setting rscript_libs = NULL disables it. Done means the inheritance and opt-out behavior are covered by the relevant tests.

Written by the indexing model from the issue text.