midipix.env:${DEFAULT_BUILD_VARS}: adds INSTALL_FILES_V2.
    subr.rtl/rtl_install_v2.subr:rtlp_install_fmap(): fix typo.
    subr.rtl/rtl_install_v2.subr:rtlp_install_fmap_patterns(): don't prepend ${_rifp2_prefix} w/ %<*>.
    subr/ex_pkg_exec.subr:exp_pkg_exec_pre(): honour ${PKG_INSTALL_FILES:-} in emitting missing_pkg.
    subr/pkg_install_files.subr:pkgp_install_files_v2(): initial implementation.