etc/build.usage, subr/build_{args,checks}.subr: merges -c into -C as -C prefix.
    etc/build.usage, subr/build_{args,init}.subr: merges -[46N] into -F as ipv4|ipv6|offline.
    subr/ex_rtl_fetch.subr: honour ${{ARG_FETCH_FORCE,DEFAULT_GIT_ARGS}}.
    subr/pkg_install_strip.subr: correctly strip non-release build binaries.
    subr/pkg_install_strip.subr:pkg_install_strip(): honour ${ARG_DEBUG_MINIPIX}.
    vars/env.vars, subr/pkg_fetch_{download,git,wget}.subr: merges fetch_{git,wget} into fetch_download.
    vars/midipix.vars: adds ${DEFAULT_GIT_ARGS}.
    vars/{musl,python[23]{,_host}}.vars: updated concerning fetch_download.