build.sh: always print std{out,err} logs given ${ARG_XTRACE} (-x.)
    etc/build.usage: updated.
    subr/ex_pkg_dispatch.subr: set -o xtrace within child process given ${ARG_XTRACE} (-x.)
    subr/ex_setup_args.subr: set ${ARG_VERBOSE2} (-vv) given ${ARG_XTRACE} (-x.)