Add label $name | EXP; fix break

This is to fix the problem where `break` is dynamic, not lexical.

With this it should be possible to do this sort of thing:

    label $break | inputs | if ... then $break|error else . end

This is a backwards-incompatible change for master, but the previous
`break` hadn't shipped yet.

Still needed:

 - testing

main.c

@@ -351,6 +351,7 @@ int main(int argc, char* argv[]) {
         options |= EXIT_STATUS;
         if (!short_opts) continue;
       }
+      // FIXME: For --arg* we should check that the varname is acceptable
       if (isoption(argv[i], 0, "arg", &short_opts)) {
         if (i >= argc - 2) {
           fprintf(stderr, "%s: --arg takes two parameters (e.g. -a varname value)\n", progname);