Add mg_http_send_redirect and make examples use it

PUBLISHED_FROM=f45e06aeba1c00212078d539b6f74cb8bd054326

docs/c-api/util.h/mg_avprintf.md

@@ -6,17 +6,5 @@ signature: |
   int mg_avprintf(char **buf, size_t size, const char *fmt, va_list ap);
 ---
 
-Prints message to the buffer. If the buffer is large enough to hold the
-message, it returns buffer. If buffer is to small, it allocates a large
-enough buffer on heap and returns allocated buffer.
-This is a supposed use case:
-
-   char buf[5], *p = buf;
-   p = mg_avprintf(&p, sizeof(buf), "%s", "hi there");
-   use_p_somehow(p);
-   if (p != buf) {
-     free(p);
-   }
-
-The purpose of this is to avoid malloc-ing if generated strings are small. 
+Same as mg_asprintf, but takes varargs list.