multissl: make openssl + wolfssl builds work

multissl: make openssl + wolfssl builds work

- make colliding vtls static function names unique.
- wolfssl: stop including an unused compatibility header.
- cmake: adapt detection logic for openssl+wolfssl coexist.
- wolfssl: fix to use native wolfSSL API in ECH codepath.
- openssl+wolfssl: fix ECH code to coexist.

Requires a post wolfSSL v5.7.4, recent master for `OPENSSL_COEXIST`
feature, and `CPPFLAGS=-DOPENSSL_COEXIST`.

Ref: https://github.com/wolfSSL/wolfssl/issues/8194

Closes #15596