pkgbse: In setting up multiple chroot areas (or the like) it would be handy to be able to download just once

In setting up multiple chroot areas (or the like) it would be
handy to be able to download just once and reuse the downloads
while setting up the various chroots --instead of hitting the
FreeBSD server with repeated downloads, for example.

The example context was setting up armv7 chroot areas on a
aarch64 system. I worked from inside each chroot area to convert
it to pkgbase.

For what I was doing, some of the pkgbase packages are not needed
( FreeBSD-src* and FreeBSD-kernel-generic* ). So only downloading
the ones used (as seen in the net result) could be part of this.
(The "net result" note is because of not being able to indicate
skipping a few: deleted later instead.)

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Mark Millard
marklmi at yahoo.com