build.sh: reintroduce {disabled,skipped}_pkg messages at -v[v[v]].
build.sh, subr/build_init.subr: minor cleanup.
subr/ex_pkg_dispatch.subr:exp_pkg_expand_packages(): call ${_dispatch_fn} w/ {disabled,skipped}_pkg for {disabled,skipped} packages.
subr/ex_pkg_dispatch.subr:exp_pkg_expand_packages(): don't mutate ${EX_PKG_NSKIPPED}.
subr/ex_pkg_dispatch.subr:ex_pkg_dispatch(): don't set ${EX_PKG_NSKIPPED}.