clean removes @ and machine now, so no need to do it again.
Retire the '@' symlink. It isn't really needed and causes more
problems than it solves. SYSDIR is already defined almost always and
can be used instead. Working around the one case where it isn't is
much easier than working around the fact that @ may not exist in 18
other places.

Differential Revision: https://reviews.freebsd.org/D1100
These don't belong in the modules directory.
Ignore errors from rm -rf to support high -j builds. This is, at best,
a kludge. However, it also effectively works around the issues for
high -j builds on systems that do not have the rm fixes.

A better fix would be to rmdir here, and fix the places where we're
sloppy and not list all the files we create in CLEANFILES, should
anybody have the time to chase them all to ground.