don't remove a.out explicitly. a.out should only be generated for
libraries and is removed in the non-default rule in bsd.lib.mk.
Removed undocumented cleanfiles target. It was the same as the
default clean target except it didn't descend into subdirs. It was
different from special clean targets in other ways. This feature
hasn't been missed for more important targets.
Removed unused default cleandepend target. bsd.dep.mk has a better
version which is always used.
Use a better rule for checkdpadd in the BINFORMAT=aout case. This
mainly checks that ld -f is working correctly. The old rule is
still available via `make BINFORMAT=foo checkdpadd' and should be
used to check for regressions under 2.2 where ld -f is not available.
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