we don't support these but advanced users may make use of them

* Attempt to build against BoringSSL in CI

* Check for BoringSSL in the SSL bindings

* Check for BoringSSL in the err bindings

* Check for BoringSSL in the pkcs7 bindings

* Check for BoringSSL in the bignum bindings

* Check for BoringSSL in the EVP bindings

* Check for BoringSSL in the X.509 verify bindings

* Add LibreSSL 3.4.0 to CI

* Add a LibreSSL 3.4.0 guard

Since LibreSSL 3.4.0 makes most of the TLSv1.3 API available, redefine CRYPTOGRAPHY_OPENSSL_LESS_THAN_111 to LibreSSL versions below 3.4.0.

* DTLS_get_data_mtu does not exist in LibreSSL

* Only EVP_Digest{Sign,Verify} exist in LibreSSL 3.4.0+

* SSL_CTX_{set,get}_keylog_callback does not exist in LibreSSL

* Do not pollute CRYPTOGRAPHY_OPENSSL_LESS_THAN_111 with LibreSSL

While LibreSSL 3.4.0 supports more of TLSv1.3 API, the guard redefinition caused the X448 tests to run when not intended.

* Expose a few more OpenSSL functions that are useful for DTLS support

* Move BIO_ADDR gunk to proper place

* const correct

* Throw more #ifdefs at the wall and see if they stick

* njsmith used "think about what he's doing"

it's probably not very effective

* LibreSSL is not my favorite library

* Attempt to hide my new undefined symbols

* deflake

* Give up on trying to check function pointers for NULLness

AFAICT it works fine in CFFI's ABI mode, but I can't figure out how to
do it in the API mode.

* LibreSSL 3.3.2 supports SSL_OP_NO_DTLS*

While here, bump CI

* Fix preprocessor guards for LibreSSL's SSL_OP_NO_DTLS*

DTLS_set_link_mtu and DTLS_get_link_min_mtu are not part of 3.3.2

* Switch to LESS_THAN context for LibreSSL 3.3.2

While here, fix indents

* Remove extra C variable declaration

The variable is not actually used from Python