Implements ${{DEFAULT,PKG,...}_MIRRORS}.
    
    build.sh:build(): define and zero-initialise ${DEFAULT_MIRRORS}.
    etc/README.md: updated.
    midipix.env: adds ${DEFAULT_MIRRORS} (https://midipix.org/mirror/ https://midipix.lucioillanes.de/archives/).
    subr.rtl/rtl_fetch.subr:rtl_fetch_url_wget(): receive and operate on target name and optional list of mirrors.
    subr/pkg_fetch_download.subr: additionally pass ${PKG_NAME} and ${PKG_MIRRORS} to rtl_fetch_url_wget().