Resolver: retry sending queries on errors (ticket #1511).

Errors when sending UDP datagrams can happen, e.g., when local IP address
changes (see fa0e093b64d7), or an unavailable DNS server on the LAN can cause
send() to fail with EHOSTDOWN on BSD systems.  If this happens during
initial query, retry sending immediately, to a different DNS server when
possible.  If this is not enough, allow normal resend to happen by ignoring
the return code of the second ngx_resolver_send_query() call, much like we
do in ngx_resolver_resend().

src/core/ngx_resolver.c

@@ -847,7 +847,15 @@ ngx_resolve_name_locked(ngx_resolver_t *r, ngx_resolver_ctx_t *ctx,
     rn->nsrvs = 0;
 
     if (ngx_resolver_send_query(r, rn) != NGX_OK) {
-        goto failed;
+
+        /* immediately retry once on failure */
+
+        rn->last_connection++;
+        if (rn->last_connection == r->connections.nelts) {
+            rn->last_connection = 0;
+        }
+
+        (void) ngx_resolver_send_query(r, rn);
     }
 
     if (ngx_resolver_set_timeout(r, ctx) != NGX_OK) {
@@ -1051,7 +1059,15 @@ ngx_resolve_addr(ngx_resolver_ctx_t *ctx)
     rn->nsrvs = 0;
 
     if (ngx_resolver_send_query(r, rn) != NGX_OK) {
-        goto failed;
+
+        /* immediately retry once on failure */
+
+        rn->last_connection++;
+        if (rn->last_connection == r->connections.nelts) {
+            rn->last_connection = 0;
+        }
+
+        (void) ngx_resolver_send_query(r, rn);
     }
 
     if (ngx_resolver_set_timeout(r, ctx) != NGX_OK) {