sl: Fix exit(0) when incorrect usage

Any command that uses lib/sl for sub-commands fails to exit with a
non-zero exit code on usage error.

The fix is a one-character change in lib/sl/slc-gram.y.

Affected are all subcommands of:

 - ktutil
 - kadmin
 - heimtools
 - hxtool
 - gsstool
 - kdigest
 - iprop-log

lib/sl/slc-gram.y

@@ -686,7 +686,7 @@ gen_wrapper(struct assignment *as)
 	(*th->free)(s);
 	free(s);
     }
-    cprint(1, "return 0;\n");
+    cprint(1, "return 1;\n");
     cprint(0, "}\n");
     cprint(0, "\n");
     free(n);