lei: support /dev/fd/[0-2] inputs and outputs in daemon
Since lei-daemon won't have the same FDs as the client, we
need to special-case thse mappings and won't be able to open
arbitrary, non-standard FDs.
We also won't attempt to support /proc/self/fd/[0-2] since
that's a Linux-ism. /dev/fd/[0-2] and /dev/std{in,out,err}
are portable to FreeBSD, at least. mawk(1) also supports
/dev/std{out,err}, as does gawk(1) (which supports everything
we can support, and arbitrary /dev/fd/$FD).