This patch moves various OpenSSL-specific function calls into the
OpenSSL module and introduces ngx_ssl_ciphers() to make nginx more
crypto-library-agnostic.

It's properly aligned and can hold any supported sockaddr.

OpenSSL 1.0.2+ allows configuring a curve list instead of a single curve
previously supported.  This allows use of different curves depending on
what client supports (as available via the elliptic_curves extension),
and also allows use of different curves in an ECDHE key exchange and
in the ECDSA certificate.

The special value "auto" was introduced (now the default for ssl_ecdh_curve),
which means "use an internal list of curves as available in the OpenSSL
library used".  For versions prior to OpenSSL 1.0.2 it maps to "prime256v1"
as previously used.  The default in 1.0.2b+ prefers prime256v1 as well
(and X25519 in OpenSSL 1.1.0+).

As client vs. server preference of curves is controlled by the
same option as used for ciphers (SSL_OP_CIPHER_SERVER_PREFERENCE),
the ssl_prefer_server_ciphers directive now controls both.

OpenSSL removed support for all 40 and 56 bit ciphers.