Exit successfully with --help

Passing --help is a legitimate use, so it should not exit(1). Besides,
this is useful to test whether the command is available.

shairport.c

@@ -140,9 +140,11 @@ int parse_options(int argc, char **argv) {
                               long_options, NULL)) > 0) {
         switch (opt) {
             default:
-            case 'h':
                 usage(argv[0]);
                 exit(1);
+            case 'h':
+                usage(argv[0]);
+                exit(0);
             case 'd':
                 config.daemonise = 1;
                 break;