manpage: fixed a variable name in the example
manpage: option descriptions use bsu_* more

an option description mentioning other values refers to them
through the config file variable names, to make the fact that
they can come from .bs-update more readily apparent.
manpage: option descriptions mention defaults

excepting -h and -n.
complex tag->version transforms using a hook

$bsu_version is derived from $bsu_tag using bsu_version_hook
if that function exists.