[nginx] SSL: support ALPN (IETF's successor to NPN).

[Piotr Sikora]

Hey Ruslan,

> This change breaks compilation with clang on systems
> where OpenSSL doesn't have ALPN support:

Good catch! Sorry for introducing this (even though it's clearly a
false positive)... I usually compile my ALPN & ecdh_auto patches
against both: OpenSSL-1.0.1 & OpenSSL-1.0.2, but I forgot to do this
after the last round of "style changes".

Best regards,
Piotr Sikora