This seems to fix the occasional "make check-run" failures I've
been chasing.
Some parts of our code assumes we can close($lei->{1})
and similar, which causes IO::Handle::autoflush to behave
badly when STDOUT is the "select"-ed FH of the Perl process.
Since oneshot mode is (hopefully) the uncommon case, we'll
just accept the cost of extra FDs and minimize differences
between lei in oneshot vs daemon mode.