diff3: Move to own package

[emaste]

Summary

diff3 is the last remaining GPL-licensed userland component.  Move it
to its own package, so that it may be excluded if desired.

[ivy]

i don't usually like a single utility being in its own package, but given the license issue this seems reasonable.

however, you did not add a UCL file for the new package :-)

[emaste]

add UCL

[ivy]

i would suggest adding licenses = [ "GPL-2.0-or-later" ]. although most of the license metadata is wrong, we should at least set it correctly for a package that only exists because of licensing issues.

also, please add a license header to the UCL file itself, i've recently updated nearly all of them to have one.

aside from that, looks fine.

[emaste]

add license for package and SPDX tag for the metadata itself