The variable contains a negotiated curve used for the handshake key
exchange process.  Known curves are listed by their names, unknown
ones are shown in hex.

Note that for resumed sessions in TLSv1.2 and older protocols,
$ssl_curve contains the curve used during the initial handshake,
while in TLSv1.3 it contains the curve used during the session
resumption (see the SSL_get_negotiated_group manual page for
details).

The variable is only meaningful when using OpenSSL 3.0 and above.
With older versions the variable is empty.

Mostly found by gcc -Wtraditional, per "non-static declaration of ...
follows static declaration [-Wtraditional]" warnings.

The directive sets the server list of supported application protocols
and requires one of this protocols to be negotiated if client is using
ALPN.

The variable contains protocol selected by ALPN during handshake and
is empty otherwise.

The "proxy_half_close" directive enables handling of TCP half close.  If
enabled, connection to proxied server is kept open until both read ends get
EOF.  Write end shutdown is properly transmitted via proxy.

To load old/weak server or client certificates it might be needed to adjust
the security level, as introduced in OpenSSL 1.1.0.  This change ensures that
ciphers are set before loading the certificates, so security level changes
via the cipher string apply to certificate loading.

Based on a patch by Anbang Wen.

With this change, it is now possible to use ngx_conf_merge_ptr_value()
to merge complex values.  This change follows much earlier changes in
ngx_conf_merge_ptr_value() and ngx_conf_set_str_array_slot()
in 1452:cd586e963db0 (0.6.10) and 1701:40d004d95d88 (0.6.22), and the
change in ngx_conf_set_keyval_slot() (7728:485dba3e2a01, 1.19.4).

To preserve compatibility with existing 3rd party modules, both NULL
and NGX_CONF_UNSET_PTR are accepted for now.

Sun C complains about "statement not reached" if a "return" is followed
by additional statements.