[3.6] bpo-30622: Change NPN detection: (GH-2079)

[tiran]

• Change NPN detection:

Version breakdown, support disabled (pre-patch/post-patch):

• pre-1.0.1: OPENSSL_NPN_NEGOTIATED will not be defined -> False/False
• 1.0.1 and 1.0.2: OPENSSL_NPN_NEGOTIATED will not be defined ->
  False/False
• 1.1.0+: OPENSSL_NPN_NEGOTIATED will be defined and
  OPENSSL_NO_NEXTPROTONEG will be defined -> True/False

Version breakdown support enabled (pre-patch/post-patch):

• pre-1.0.1: OPENSSL_NPN_NEGOTIATED will not be defined -> False/False
• 1.0.1 and 1.0.2: OPENSSL_NPN_NEGOTIATED will be defined and
  OPENSSL_NO_NEXTPROTONEG will not be defined -> True/True
• 1.1.0+: OPENSSL_NPN_NEGOTIATED will be defined and
  OPENSSL_NO_NEXTPROTONEG will not be defined -> True/True

• Refine NPN guard:

• If NPN is disabled, but ALPN is available we need our callback
• Make clinic's ssl behave the same way

This created a working ssl module for me, with NPN disabled and ALPN
enabled for OpenSSL 1.1.0f.

Concerns to address:
The initial commit for NPN support into OpenSSL [1], had the
OPENSSL_NPN_* variables defined inside the OPENSSL_NO_NEXTPROTONEG
guard. The question is if that ever made it into a release.
This would need an ugly hack, something like:

GH-if defined(OPENSSL_NO_NEXTPROTONEG) && \
	!defined(OPENSSL_NPN_NEGOTIATED)
GH-	define OPENSSL_NPN_UNSUPPORTED 0
GH-	define OPENSSL_NPN_NEGOTIATED 1
GH-	define OPENSSL_NPN_NO_OVERLAP 2
GH-endif

[1] openssl/openssl@68b33cc5c7
(cherry picked from commit b2d096b)

https://bugs.python.org/issue30622

[Mariatta]

adding skip news here is fine :) If you're so inclined, you can prepare another PR with the news item.