makefs enhancement for better rock-ridge support

A while ago, it was reported that the ISO images that FreeBSD generates
have a variety of problems (thread starts here):

http://lists.freebsd.org/pipermail/freebsd-stable/2013-April/073050.html

And again for the 10.0 releases:

http://lists.freebsd.org/pipermail/freebsd-stable/2013-December/076284.html

Looking into this, it appears that the various bugs in the Rock Ridge
extensions have been fixed, except for the actual lack of recording
the "serial" numbers in the correct place of the Rock Ridge data.

As it turns out, it is almost trivial to fix this.

Patch is attached to this message, which will probably be stripped
out by the mailing list, but should be available as an attachment
from the mail server.

-Kurt