* Fix weak linking of getentropy when compiling on older macOS

We use weak linking in macOS to determine if the getentropy symbol is
available. However, to do that we need to have a declaration that states
the function is __attribute((weak_import)) at compile time. On macOS
10.12 this is provided in sys/random.h, but on older macOS the
declaration doesn't exist at all, so we need to forward declare it
ourselves.

* update a comment and a style nit

the getentropy fallback is only possible on macOS, wrap it in a define
to remove it entirely on the BSDs.

* allow p % 24 == 23 when generator == 2 in DH_check

* short url

* update and expand comments

* even better language!

* remove egd

* oops

* keep Cryptography_HAS_EGD for compat just in case

This shouldn't really be necessary but maybe we can fully remove it in
2018 or 2019...

* remove cryptodev

* oops

* Remove conditionals we never use.

Refs #3763

* put this back

* No more FUNCS/MACROS distinction

* change the docs to not talk about MACROS since they're gone

* remove out of date comment

* enable wconversion and finish fixes

* don't pass -Wconversion if it's win32

* bind DTLS 1.2 methods

* remove version specific dtls bindings, rename sentinel value