Restore Referer-Policy and wrap up MbedTLS changes

redbean will now set Referer-Policy to no-referrer-when-downgrade on
text/html responses by default. There's better explanations on the bits
of security redbean is offering. In short, it's 128+ for modern clients
and 112+ for legacy. If the -B flag is used then it's 192+ for modern
and 150+ for non-EC.

libc/stdio/appendd.c

@@ -44,7 +44,7 @@ int appendd(char **b, const void *s, size_t l) {
       return -1;
     }
   }
-  memcpy(p + z.i, s, l + 1);
+  *(char *)mempcpy(p + z.i, s, l) = 0;
   z.i += l;
   if (!IsTiny() && W == 8) {
     z.i |= (size_t)APPEND_COOKIE << 48;