Initial implementation of pkgtool.sh.
    
    build.sh:build(): move cd(1) to $(dirname "${0}").
    etc/{build.usage,README.md}: updated.
    midipix.env:${DEFAULT_CLEAR_ENV_VARS_EXCEPT}: allow inheriting ARCH, BUILD, BUILD_DLCACHEDIR, BUILD_WORKDIR, and PREFIX* from the environment.
    pkgtool.sh: initial implementation.
    subr/build_init.subr:buildp_init_defaults(): allow inheriting ARCH, BUILD, BUILD_DLCACHEDIR, BUILD_WORKDIR, and PREFIX* from the environment.
    subr/build_init.subr:buildp_init_env(): move cd(1) to $(dirname "${0}").
    subr/build_init.subr:buildp_init_files(): correctly pass ${_status} from rtl_check_path_vars().
    subr/ex_pkg_exec.subr:ex_pkg_exec(): dump subset of variables and exported variables on build failure and --dump-on-abort.
    subr/pkgtool_init.subr: adapted from subr/build_init.subr.
    subr/rtl_complex.subr:rtl_filter_vars(): initial implementation.
    subr/rtl_string.subr:rtl_subst(): initial implementation.