Rename to fit with the other thread names.

Before this change:

hostname:/proc/24139$ cat task/*/comm | sort | uniq
Backend poller
ban-lurker
cache-acceptor
cache-epoll
cache-main
cache-timeout
cache-worker
hcb-cleaner
MPL_busyobj
MPL_req0
MPL_req1
MPL_sess0
MPL_sess1
pool_herder
vsm_cleaner

bin/varnishd/cache/cache_backend_probe.c

@@ -627,5 +627,5 @@ VBP_Init(void)
 	vbp_heap = binheap_new(NULL, vbp_cmp, vbp_update);
 	AN(vbp_heap);
 	AZ(pthread_cond_init(&vbp_cond, NULL));
-	WRK_BgThread(&thr, "Backend poller", vbp_thread, NULL);
+	WRK_BgThread(&thr, "backend-poller", vbp_thread, NULL);
 }