Implements setting up & fetching from mirrors.
    
    etc/pkgtool.usage: updated.
    etc/README.md: updated.
    pkgtool.sh:pkgtoolp_mirror{,_fetch}(): initial implementation.
    subr/pkg_fetch_download.subr: default to empty ${PKG_MIRRORS}.
    subr/pkg_fetch_download.subr: factor out ${ARG_FETCH_FORCE} check and ${BUILD_DLCACHEDIR}.
    subr/pkgtool_init.subr: updated.
    subr.rtl/rtl_fetch.subr:rtl_fetch_urls_{git,wget}(): factor out ${ARG_FETCH_FORCE} check and ${BUILD_DLCACHEDIR}; minor cleanup.
    subr.rtl/rtl_fetch.subr:rtl_fetch_url_wget(): return vs. exit on fatal failure.
    subr.rtl/rtl_fetch.subr:rtlp_fetch_url_git(): copy only if ${_cache_dname} != ${_tgtdir}.
    subr.rtl/rtl_fetch.subr:rtlp_fetch_url_git(): return if unable to git-clone(1).
    subr.rtl/rtl_fetch.subr:{rtlp_fetch_url_git,rtl_fetch_urls_git}(): implement Git repository mirroring.
    vars/{gcc,python[23]{,_host}}.vars: updated.