This flag appeared in Linux 4.5 and is useful for avoiding thundering herd
problem.

The current Linux kernel implementation walks the list of exclusive waiters,
and queues an event to each epfd, until it finds the first waiter that has
threads blocked on it via epoll_wait().

Now it is believed that the accept mutex brings more harm than benefits.
Especially in various benchmarks it often results in situation where only
one worker grabs all connections.

Also, removed practically unused flag accept_context_updated from
ngx_connection_t.

The option is only set if the socket is bound to a specific port to allow
several such sockets coexist at the same time.  This is required, for example,
when nginx acts as a transparent proxy and receives two datagrams from the same
client in a short time.

The feature is only implemented for Linux.

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.

Using the same DH parameters on multiple servers is believed to be subject
to precomputation attacks, see http://weakdh.org/.  Additionally, 1024 bits
are not enough in the modern world as well.  Let users provide their own
DH parameters with the ssl_dhparam directive if they want to use EDH ciphers.

Note that SSL_CTX_set_dh_auto() as provided by OpenSSL 1.1.0 uses fixed
DH parameters from RFC 5114 and RFC 3526, and therefore subject to the same
precomputation attacks.  We avoid using it as well.

This change also fixes compilation with OpenSSL 1.1.0-pre5 (aka Beta 2),
as OpenSSL developers changed their policy after releasing Beta 1 and
broke API once again by making the DH struct opaque (see ticket #860).