rpki-client: change "asn" from string to integer in JSON output

[Job Snijders]

I'd like to modify our JSON format, many people in the community have
voiced complaints that transforming the string to an integer is
annoying.

This won't break existing deployments coupled with GoRTR.

OK?

Index: output-json.c
===================================================================
RCS file: /cvs/src/usr.sbin/rpki-client/output-json.c,v
retrieving revision 1.15
diff -u -p -r1.15 output-json.c
--- output-json.c       8 Apr 2021 19:49:27 -0000       1.15
+++ output-json.c       5 May 2021 15:29:15 -0000
@@ -100,7 +100,7 @@ output_json(FILE *out, struct vrp_tree *
 
                ip_addr_print(&v->addr, v->afi, buf, sizeof(buf));
 
-               if (fprintf(out, "\t\t{ \"asn\": \"AS%u\", \"prefix\": \"%s\", "
+               if (fprintf(out, "\t\t{ \"asn\": %u, \"prefix\": \"%s\", "
                    "\"maxLength\": %u, \"ta\": \"%s\" }",
                    v->asid, buf, v->maxlength, v->tal) < 0)
                        return -1;