vsm: Centralize VARNISH_DEFAULT_N fallback

Putting the new fallback next to the historical fallback.

Refs #4105

bin/varnishadm/varnishadm.c

@@ -467,7 +467,6 @@ main(int argc, char * const *argv)
 		printf(OPTARG "\n");
 		exit(0);
 	}
-	n_arg = getenv("VARNISH_DEFAULT_N");
 	/*
 	 * By default linux::getopt(3) mangles the argv order, such that
 	 *	varnishadm -n bla param.set foo -bar

lib/libvarnishapi/vsm.c

@@ -346,8 +346,6 @@ VSM_New(void)
 	ALLOC_OBJ(vd, VSM_MAGIC);
 	AN(vd);
 
-	REPLACE(vd->wdname, getenv("VARNISH_DEFAULT_N"));
-
 	vd->mgt = vsm_newset(VSM_MGT_DIRNAME);
 	vd->mgt->flag_running = VSM_MGT_RUNNING;
 	vd->mgt->flag_changed = VSM_MGT_CHANGED;
@@ -756,6 +754,9 @@ VSM_Attach(struct vsm *vd, int progress)
 	else
 		t0 = VTIM_mono() + vd->patience;
 
+	if (vd->wdname == NULL)
+		REPLACE(vd->wdname, getenv("VARNISH_DEFAULT_N"));
+
 	if (vd->wdname == NULL) {
 		/* Use default (hostname) */
 		i = VSM_Arg(vd, 'n', "");

lib/libvarnishapi/vut.c

@@ -263,7 +263,6 @@ VUT_Init(const char *progname, int argc, char * const *argv,
 	AZ(vut->vsl);
 	vut->vsl = VSL_New();
 	AN(vut->vsl);
-	REPLACE(vut->n_arg, getenv("VARNISH_DEFAULT_N"));
 	return (vut);
 }