Implements extended -r syntax.
    
    build.sh:buildp_ast(): only reset package build step state if ${ARG_RESET_PKG} is 1.
    etc/{build.usage,README.md}: updated.
    etc/build.usage.short, subr/build_init.subr: implement short help screen.
    midipix.env: correctly escape multi-line variable assignment expressions.
    subr.rtl/rtl_list.subr: updated.
    subr.rtl/rtl_state.subr:rtl_state_set(): correctly implement invalidation.
    subr.rtl/rtl_state.subr:rtl_state_test(): honour `LAST'.
    subr/build_init.subr:buildp_init_getopts(): implements --reset-pkg.
    subr/build_init.subr:buildp_{expand_restart_{at,recursive},process_restart_spec}(): initial implementation.
    subr/ex_pkg_{dispatch,env}.subr: updated.
    subr/ex_pkg_exec.subr:ex_pkg_exec(): honour `ALL' and `LAST'.
    vars/gcc.vars: updated.