lei: less error-prone FD mapping

lei: less error-prone FD mapping

Keeping track of non-standard FDs gets tricky, so make it easier
by relying on st_dev/st_ino mapping in the transmitted objects.

We'll keep using numbers for the standard FDs since we need to
be able to easily redirect them in the producer (main daemon)
process for (gzip|bzip2|xz) if writing to a compressed mbox.