The patch saves one EC_KEY_generate_key() call per server{} block by
informing OpenSSL about SSL_OP_SINGLE_ECDH_USE we are going to use before
the SSL_CTX_set_tmp_ecdh() call.

For a configuration file with 10k simple server{} blocks with SSL enabled
this change reduces startup time from 18s to 5s on a slow test box here.

Input filter might free a buffer if there is no data in it, and in case
of first buffer (used for cache header and request header, aka p->buf_to_file)
this resulted in cache corruption.  Buffer memory was reused to read upstream
response before headers were written to disk.

Fix is to avoid moving pointers in ngx_event_pipe_add_free_buf() to a buffer
start if we were asked to free a buffer used by p->buf_to_file.

This fixes occasional cache file corruption, usually resulted
in "cache file ... has md5 collision" alerts.

Reported by Anatoli Marinov.

This parameter allows to don't require certificate to be signed by
a trusted CA, e.g. if CA certificate isn't known in advance, like in
WebID protocol.

Note that it doesn't add any security unless the certificate is actually
checked to be trusted by some external means (e.g. by a backend).

Patch by Mike Kazantsev, Eric O'Connor.

With the "ssl_stapling_verify" commit build with old OpenSSL libraries
was broken due to incorrect prototype of the ngx_ssl_stapling() function.
One incorrect use of ngx_log_debug() instead of ngx_log_debug2() slipped in
and broke win32 build.

OCSP response verification is now switched off by default to simplify
configuration, and the ssl_stapling_verify allows to switch it on.

Note that for stapling OCSP response verification isn't something required
as it will be done by a client anyway.  But doing verification on a server
allows to mitigate some attack vectors, most notably stop an attacker from
presenting some specially crafted data to all site clients.

This is expected to simplify configuration in a common case when OCSP
response is signed by a certificate already present in ssl_certificate
chain.  This case won't need any extra trusted certificates.

It's hard to debug OCSP_basic_verify() failures without the actual error
string it records in the error data field.

This will result in better error message in case of incorrect response
from OCSP responder:

... OCSP responder sent invalid "Content-Type" header: "text/plain"
    while requesting certificate status, responder: ...

vs.

... d2i_OCSP_RESPONSE() failed (SSL:
    error:0D07209B:asn1 encoding routines:ASN1_get_object:too long
    error:0D068066:asn1 encoding routines:ASN1_CHECK_TLEN:bad object header
    error:0D07803A:asn1 encoding routines:ASN1_ITEM_EX_D2I:nested asn1 error)
    while requesting certificate status, responder: ...

This includes the ssl_stapling_responder directive (defaults to OCSP
responder set in certificate's AIA extension).

OCSP response for a given certificate is requested once we get at least
one connection with certificate_status extension in ClientHello, and
certificate status won't be sent in the connection in question.  This due
to limitations in the OpenSSL API (certificate status callback is blocking).

Note: SSL_CTX_use_certificate_chain_file() was reimplemented as it doesn't
allow to access the certificate loaded via SSL_CTX.