Implements {fetch,configure,build,install}_clean.
    
    build.sh:buildp_dispatch_fail_pkg(): print NL when writing to ${DEFAULT_BUILD_LAST_FAILED_PKG_FNAME}.
    etc/{README.md,build.usage}: updated.
    midipix.env: updated.
    subr.rtl/rtl_complex.subr:rtl_sunset(): initial implementation.
    subr.rtl/rtl_list.subr:rtl_l{filter[23],length2,lift[23]}(): initial implementation.
    subr.rtl/rtl_state.subr:rtl_state_test(): take ${_build_steps} vs. ${_build_step}.
    subr/{build_init,ex_pkg_restart}.subr: splits extended restart syntax processing logic.
    subr/ex_pkg_exec.subr:ex_pkg_exec(): filter "start".
    subr/pkg_{build,configure,fetch,install}_clean.subr: initial implementation.