dhcpcd: Redirect stdout/stderr to the launcher stderr descriptor

dhcpcd: Redirect stdout/stderr to the launcher stderr descriptor

This actually make life really simple!
We no longer need to redirect stdout/stderr to /dev/null for privsep
and any script output is now captured again - and it all goes to stderr
as it should even if a script wants it to go to stdout.

On the happy path, only the master process will actually log anything
to stderr so we turn that off after we "fork".
On the unhappy path, logging to stderr/stdout *may* fail because
the launcher process *may* have exited.
We *could* have the master process as an intermediary but that's
just excess code to avoid errors which *should* not happen.
Regardless, any errror should still hit syslog.