process: move POSIX credential accessors into node_credentials.cc

Expose the POSIX credential accessors through
`internalBinding('credentials')` instead of setting them on the
process or bootstrapper object from C++ directly. Also moves
`SafeGetEnv` from `internalBinding('util')` to
`internalBinding('credentials')` since it's closely related to
the credentials.

In the JS land, instead of wrapping the bindings then writing
to the process object directly in main_thread_only.js, return
the wrapped functions back to bootstrap/node.js where they get
written to the process object conditionally for clarity.

Refs: https://github.com/nodejs/node/issues/24961

PR-URL: https://github.com/nodejs/node/pull/25066


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>