Make --cleanbuild remove all "work" files

Review Request #127192 - Created Feb. 26, 2016 and updated

Thomas Friedrichsmeier

Not sure, whether this is actually intended (strictly speaking, it does match the terminology used in emerge), but it did come as a (negative) surprise to me:
emerge --cleanbuild XY
does not clean everything in build\category\XY\work, but only the "real" build directory (with the compiler / release type specification) in there. This is ok for most (not all) source packages, but for other packages it means leaving a lot of cruft.

If this is actually the intended behavior, I'd want a --cleanwork action, in addition to this. Assuming the intended behavior is to clean everything in build\category\XY\work, this patch will do that.

Patch also makes failed removals non-fatal, in case there are write-protected files, or some-such.

Tested locally, and cleans the "work" directories for packages such as mysql-pkg, aspell, 7zip, etc., that were not cleaned, before.