遵循 DPD-REQ 协议，修复 OpenConnect DTLS 的 MTU 探测

[itviewer]

https://datatracker.ietf.org/doc/html/draft-mavrogiannopoulos-openconnect-04#name-the-cstp-channel-protocol

DPD-REQ: used for dead peer detection. Once sent the peer should reply with a DPD-RESP packet, that has the same contents as the original request.

openconnect 的 DTLS 根据 DPD-RESP 最终确定 mtu，否则设为最小值。 之前的代码没有遵循上面的描述返回原样内容，导致 openconnect 的 dtls 网速很慢

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (3f07224) 18.99% compared to head (ffd68d6) 18.98%.

Files	Patch %	Lines
server/handler/link_dtls.go	0.00%	2 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #285 +/- ## ========================================== - Coverage 18.99% 18.98% -0.01% ========================================== Files 64 64 Lines 4265 4266 +1 ========================================== Hits 810 810 - Misses 3330 3331 +1 Partials 125 125 ```

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