Report dlerror if dlopen fails

dlopen typically only fails here if the child process does not have
access to the build directory and the user runs varnishtest as root
(meaning the child setuids to nobody).  Report the dlerror and give a
hopefully helpful hint to help diagnose the error.

Fixes: #959

bin/varnishd/cache_vrt_vmod.c

@@ -83,7 +83,11 @@ VRT_Vmod_Init(void **hdl, void *ptr, int len, const char *nm, const char *path)
 		REPLACE(v->path, path);
 
 		v->hdl = dlopen(v->path, RTLD_NOW | RTLD_LOCAL);
-		AN(v->hdl);
+		if (! v->hdl) {
+			char buf[1024];
+			sprintf(buf, "dlopen failed (child process lacks permission?): %.512s", dlerror());
+			VAS_Fail(__func__, __FILE__, __LINE__, buf, 0, 0);
+		}
 
 		x = dlsym(v->hdl, "Vmod_Name");
 		AN(x);