fix: Ensure HTTP Host Header format compliance

[sfiction]

• Non-default ports are explicitly included in the Host header
• Default ports (443 for HTTPS) are omitted
• IPv6 addresses are properly enclosed in square brackets ([ ]) per RFC 3986

smartdns 在发送 https 请求时，会添加 Host 字段，内容为 https_flag->httphost。上游为 flag_http->httphost => server->httphost => server->server。server->server 则来自配置文件中服务器地址的解析结果，当 url 中 host 为 ipv6 地址时，util.c:parse_ip 会移除首尾的 [ ]。而在 RFC 规范 中，Host 字段中的 ipv6 地址必须用方括号包裹。cloudflare 和 google 的服务器都会拒绝错误格式的 Host 字段。目前可以用 -http-host 选项暂时解决这个问题。

配置：

# cloudflare
server-https https://[2606:4700:4700::64]/dns-query
# google
server-https https://[2001:4860:4860::64]/dns-query

报错：

dns_client.c:2876] http server query from 2606:4700:4700::64:443 failed, server return http code : 403, Forbidden
dns_client.c:2876] http server query from 2001:4860:4860::64:443 failed, server return http code : 400, Bad Request

其他复现方式：

# www.google.com AAAA
curl -v 'https://[2606:4700:4700::64]:443/dns-query?dns=IlkBAAABAAAAAAAAA3d3dwZnb29nbGUDY29tAAAcAAE' -H 'Host: 2606:4700:4700::64'
curl -v 'https://[2001:4860:4860::64]:443/dns-query?dns=IlkBAAABAAAAAAAAA3d3dwZnb29nbGUDY29tAAAcAAE' -H 'Host: 2001:4860:4860::64'