lei q: mbox*: disable changing parallelism, add --rsyncable
Unfortunately, being mairix-compatible with --threads means we
can't change thread-count of gzip, bzip2, or xz when writing to
compressed mbox with a --threads= parameter. It's probably not
worth changing, anyways, so another switch or additional value
for --jobs= won't be added.
While we're in the area, add --rsyncable support since
most installations of gzip support it nowadays.
Fixes: 5beb4a5f6585acd ("lei: replace --thread with --threads")