On some platforms (for example, Linux with glibc 2.12-2.25) IPv4 transparent
proxying is available, but IPv6 transparent proxying is not.  The entire feature
is enabled in this case and NGX_HAVE_TRANSPARENT_PROXY macro is set to 1.
Previously, an attempt to enable transparency for an IPv6 socket was silently
ignored in this case and was usually followed by a bind(2) EADDRNOTAVAIL error
(ticket #1487).  Now the error is generated for unavailable IPv6 transparent
proxy.

In macOS 10.12, CLOCK_REALTIME and clockid_t were added, but not timer_t.

This header carries the definition of HMAC_Init_ex(). In OpenSSL this
header is included by <openssl/ssl.h>, but it's not so in BoringSSL.

It's probably a good idea to explicitly include this header anyway,
regardless of whether it's included by other headers or not.

Some OSes (notably macOS, NetBSD, and Solaris) allow unix socket addresses
larger than struct sockaddr_un.  Moreover, some of them (macOS, Solaris)
return socklen of the socket address before it was truncated to fit the
buffer provided.  As such, on these systems socklen must not be used without
additional check that it is within the buffer provided.

Appropriate checks added to ngx_event_accept() (after accept()),
ngx_event_recvmsg() (after recvmsg()), and ngx_set_inherited_sockets()
(after getsockname()).

We also obtain socket addresses via getsockname() in
ngx_connection_local_sockaddr(), but it does not need any checks as
it is only used for INET and INET6 sockets (as there can be no
wildcard unix sockets).

The $ssl_server_name variable used SSL_get_servername() result directly,
but this is not safe: it references a memory allocation in an SSL
session, and this memory might be freed at any time due to renegotiation.
Instead, copy the name to memory allocated from the pool.

This variable contains URL-encoded client SSL certificate.  In contrast
to $ssl_client_cert, it doesn't depend on deprecated header continuation.
The NGX_ESCAPE_URI_COMPONENT variant of encoding is used, so the resulting
variable can be safely used not only in headers, but also as a request
argument.

The $ssl_client_cert variable should be considered deprecated now.
The $ssl_client_raw_cert variable will be eventually renambed back
to $ssl_client_cert.

The "fd" field should be after 3 pointers for ngx_event_ident() to use it.
This was broken by ccad84a174e0.  While it does not seem to be currently used
for aio-related events, it should be a good idea to preserve the correct
layout nevertheless.

Pass NGX_FILE_OPEN to ngx_open_file() to fix "The parameter is incorrect"
error on win32 when using the ssl_session_ticket_key directive or loading
a binary geo base.  On UNIX, this change is a no-op.