build.sh:build(): iteratively print list of unknown packages, if any.
    subr/build_init.subr:buildp_init_args(): correctly parse -r package[,...]:step[,...].
    subr/ex_pkg{,_dispatch}.subr: minor cleanup.
    subr/ex_pkg_dispatch.subr:ex_pkg_dispatch(): don't reset ${EX_PKG_DISPATCH_UNKNOWN} on entry.
    subr/ex_pkg_dispatch.subr:ex_pkg_dispatch(): correctly check ${_pkg_names} for ${_pkg_name} when updating ${EX_PKG_DISPATCH_UNKNOWN}.