fix DTLS header headroom accounting

[rizlik]

Description

Refactor wolfSSL_GetMaxFragSize() to return just the Max frag size, and use the more explicit wolfSSL_GetMaxPlaintextSize() to get max plaintext in building message.

Simplify lengths calculation and fix over-estimation of DTLS headers headroom.

closes #8939

[devin-ai-integration]

🛟 Devin Lifeguard found 1 likely issues in this PR

• pointer-null-check snippet snippet snippet: Add if (ssl == NULL) return BAD_FUNC_ARG; (or appropriate error) at the start of wolfSSL_GetRecordSize, wolfSSL_GetMaxPlaintextSize, and wolfSSL_GetMaxFragSize before any dereference of ssl.

@rizlik
please take a look at the above issues which Devin flagged. Devin will not fix these issues automatically.

[rizlik]

retest this please