build.sh:fail_pkg: terminate pending builds given build failure, -P, and absence of -R.
    etc/build.usage: clarify the consequences of parallelising builds to guide the perplexed and to confuse the people of the tribe of certainty into enlightenment (or not?)
    subr/ex_pkg_dispatch.subr:exp_pkg_dispatch(): moved traps.
    subr/ex_pkg_exec.subr:ex_pkg_exec(): forcibly pkill(1) child processes on failure.