makefs Sparse Files: NetBSD CLI Compatibility

[Eric van Gyzen]

NetBSD's makefs has a -Z flag to create the image as a sparse file.  In
FreeBSD, the flag is spelled -p.  Is there a reason for using a
different flag?  It would be very nice to preserve CLI compatibility
with NetBSD.

NetBSD committed first (by one month), and neither change has gone into
a release yet, so we should change to match NetBSD.  We should do it
soon, too, since our change will go into 9.2-RELEASE.

If we agree, I'll gladly make the patches, trivial though they'll be.

Eric