fs: expose realpath(3) bindings

Make the `uv_fs_realpath()` binding (which calls the libc `realpath()`
on UNIX and `GetFinalPathNameByHandle()` on Windows) available as the
`fs.realpath.native()` and `fs.realpathSync.native()` functions.

The binding was already available as `process.binding('fs').realpath`
but was not exposed or tested - and partly broken as a result.

Fixes: https://github.com/nodejs/node/issues/8715
PR-URL: https://github.com/nodejs/node/pull/15776
Refs: https://github.com/nodejs/node/pull/7899


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>