I'm looking at this as the Guix Data Service uses this behaviour to record and
display logs from inferior processes.
* guix/inferior.scm (open-bidirectional-pipe): Call dup2 for file descriptor
2, passing either the file number for the current error port, or a file
descriptor for /dev/null.
* tests/inferior.scm ("#:error-port stderr", "#:error-port pipe"): Add two new
tests that cover some of the #:error-port behaviour.
