guestfish: Redirect stdout when executing remote commands

guestfish: Redirect stdout when executing remote commands

guestfish --listen necessarily redirects its stdout to /dev/null so as not to
interfere with eval. The remote protocol doesn't contain any other provision for
collecting stdout for the caller, so executing guestfish --remote will never
generate any output.

This patch fixes that by forwarding the caller's STDOUT to the listener over the
unix socket connection. The listener redirects its STDOUT to the caller's STDOUT
for the duration of the command, then closes it again.