(2) restructure machine dependencies into separate chapter
(3) replace pretty, but quick to become obsolete, graph of BFD platforms
vs architectures with extended excerpt from (and ref to ) objdump -i.
description believed complete as of today; still holes---small
ones---in "commands" section. BFD section also needs better table of
supported archs and formats; see FIXMEs for scattered q's and
desiderata.