* 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.

* Fixed #5050 -- dropped support for an old LibresSSL release

* Changelog

In #4220, I switched BIO_new to take a const pointer (true in 1.1.0, but
not 1.0.2) on grounds that:

    This also folds in the const bits from 1.1.0, on the assumption that,
    now that the function pointer check is gone, it will just cause cffi to
    generate more conservative pointer types that work for 1.0.2 as well.

But I got this backwards. If the bindings say BIO_METHOD*, cffi will
pass a BIO_METHOD* to BIO_new, which works in both OpenSSL versions. If
it says const BIO_METHOD*, cffi will pass const BIO_METHD* to BIO_new,
which does not work in 1.0.2.

(Although cryptography.io's build ignores all these warnings anyway, so
it's kind of moot.)

* libre 2.7.3 compatibility

* add a changelog

* actually build against 2.7.3

* Remove defines for openssl 1.1.0 pre

* Update bio.py

* Update dh.py

* Update dsa.py

* Update rsa.py

* Update x509_vfy.py

* Compress branches