Use newer mkusage syntax and usage() convention

bin/mkx

             echo 'shellfu import pretty'
             echo ''
             echo 'usage() {'
-            echo '    mkusage "arg..."'
+            echo '    mkusage "$@" "[-d] [-v] ARG..."'
             echo '}'
             echo ''
             echo 'main() {'
             echo '    while true; do case $1 in'
             echo '        -d) PRETTY_DEBUG=true;   shift ;;'
             echo '        -v) PRETTY_VERBOSE=true; shift ;;'
-            echo '        -*) usage ;;'
+            echo '        -*) usage -w "unknown argument: $1" ;;'
             echo '        *)  break ;;'
             echo '    esac done'
             echo '}'