Make targets: improve *clean, remove bin, cvs-clean and maintainerclean

Make targets: improve *clean, remove bin, cvs-clean and maintainerclean

* distclean was usable from any directory in theory, make it
toplevel-only and call clean, doc-clean, test-clean and log-clean
(previous behaviour can now be obtained with make clean && make
local-distclean),

* remove maintainerclean, which was a broken distclean-like, and
cvs-clean that can be substituted by some "git clean" incantation,

* remove bin, which does not work in a clean tree, and that can be
replaced by  make bin-clean && make all in an already built tree,

* update help target.