inferior: Allow controlling the inferior error port.

inferior: Allow controlling the inferior error port.

Previously, stderr for the inferior process would always be sent to /dev/null
because the current-output-port when the process is launched is a void
port. This change means that it's possible to pass in a different port to use.

* guix/inferior.scm (inferior-pipe): Take the error-port as an argument.
(open-inferior): Add new error-port keyword argument, with a default
of (%make-void-port "w").