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.

In 71edd9192f24 logging of invalid headers which were rejected with the
NGX_HTTP_PARSE_INVALID_HEADER error was restricted to just the "client
sent invalid header line" message, without any attempts to log the header
itself.

This patch returns logging of the header up to the invalid character and
the character itself.  The r->header_end pointer is now properly set
in all cases to make logging possible.

The same logging is also introduced when parsing headers from upstream
servers.

Control characters (0x00-0x1f, 0x7f), space, and colon were never allowed in
header names.  The only somewhat valid use is header continuation which nginx
never supported and which is explicitly obsolete by RFC 7230.

Previously, such headers were considered invalid and were ignored by default
(as per ignore_invalid_headers directive).  With this change, such headers
are unconditionally rejected.

It is expected to make nginx more resilient to various attacks, in particular,
with ignore_invalid_headers switched off (which is inherently unsecure, though
nevertheless sometimes used in the wild).

From now on, requests with spaces in URIs are immediately rejected rather
than allowed.  Spaces were allowed in 31e9677b15a1 (0.8.41) to handle bad
clients.  It is believed that now this behaviour causes more harm than
good.

After 2096b21fcd10, a single RST_STREAM(NO_ERROR) may not result in an error.
This change removes several unnecessary ctx->type checks for such a case.

Previously, once received from upstream, it couldn't limit
opening additional streams in a cached keepalive connection.

The header is escaped in redirects based on request URI or
location name (auto redirect).

This saves some memory in typical case when auth_basic_user_file is not
explicitly set, and unifies the code with alcf->realm.

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.