This is a partial revert of nodejs@b488b19
It restores old javascript implementation of realpath and realpathSync.
The names have been changed: added JS and not exported in fs module.

Before v6 on drives created with subst and network mapped drives
realpath used to return filename on the mapped drive. With v6 it now
returns filename on the original device or on the network share. This
restores the old behavior by using old javascript implementation when
path returned by new implementation is on a different device than the
original path.

Fixes: 7294