cmd/midx: add a new --max-files parameter.

cmd/midx: add a new --max-files parameter.

Zoran reported that 'bup midx -f' on his system tried to open 3000 files at
a time and wouldn't work.  That's no good, so let's limit the maximum files
to open; the default is 500 for now, since that ought to be usable for
normal people.  Arguably we could use getrlimit() or something to find out
the actual maximum, or just keep opening stuff until we get an error, but
maybe there's no point.

Unfortunately this patch isn't really perfect, because it limits the
usefulness of midx files.  If you could merge midx files into other midx
files, then you could at least group them all together after multiple runs,
but that's not currently supported.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>