community/{dar,libthreadar}: move from testing

libthreadar is a dependency of dar - it's a separate package as it is maintained as a separate upstream project and has a release schedule independent of dar. At this time only dar depends on the libthreadar package in Alpine. Hence I propose to move both of these to community as a unit.

I hereby pledge to continue to maintain these packages and also to submit any security fixes for these packages into the alpine release branches.