Passwords were not preserved in optimized SSL contexts, the bug had
appeared in d791b4aab (1.23.1), as in the following configuration:

    server {
        proxy_ssl_password_file password;
        proxy_ssl_certificate $ssl_server_name.crt;
        proxy_ssl_certificate_key $ssl_server_name.key;

        location /original/ {
            proxy_pass https://u1/;
        }

        location /optimized/ {
            proxy_pass https://u2/;
        }
    }

The fix is to always preserve passwords, by copying to the configuration
pool, if dynamic certificates are used.  This is done as part of merging
"ssl_passwords" configuration.

To minimize the number of copies, a preserved version is then used for
inheritance.  A notable exception is inheritance of preserved empty
passwords to the context with statically configured certificates:

    server {
        proxy_ssl_certificate $ssl_server_name.crt;
        proxy_ssl_certificate_key $ssl_server_name.key;

        location / {
            proxy_pass ...;

            proxy_ssl_certificate example.com.crt;
            proxy_ssl_certificate_key example.com.key;
        }
    }

In this case, an unmodified version (NULL) of empty passwords is set,
to allow reading them from the password prompt on nginx startup.

As an additional optimization, a preserved instance of inherited
configured passwords is set to the previous level, to inherit it
to other contexts:

    server {
        proxy_ssl_password_file password;

        location /1/ {
            proxy_pass https://u1/;
            proxy_ssl_certificate $ssl_server_name.crt;
            proxy_ssl_certificate_key $ssl_server_name.key;
        }

        location /2/ {
            proxy_pass https://u2/;
            proxy_ssl_certificate $ssl_server_name.crt;
            proxy_ssl_certificate_key $ssl_server_name.key;
        }
    }

It was possible to write outside of the buffer used to keep UTF-8
decoded values when parsing conversion table configuration.

Since this happened before UTF-8 decoding, the fix is to check in
advance if character codes are of more than 3-byte sequence.  Note
that this is already enforced by a later check for ngx_utf8_decode()
decoded values for 0xffff, which corresponds to the maximum value
encoded as a valid 3-byte sequence, so the fix does not affect the
valid values.

Found with AddressSanitizer.
Fixes GitHub issue #529.

As uncovered by recent addition in slice.t, a partially initialized
context, coupled with HTTP 206 response from stub backend, might be
accessed in the next slice subrequest.

Found by bad memory allocator simulation.

Caching is enabled with proxy_ssl_certificate_cache and friends.

Co-authored-by: Aleksei Bavshin <a.bavshin@nginx.com>

A new directive "ssl_certificate_cache max=N [valid=time] [inactive=time]"
enables caching of SSL certificate chain and secret key objects specified
by "ssl_certificate" and "ssl_certificate_key" directives with variables.

Co-authored-by: Aleksei Bavshin <a.bavshin@nginx.com>

It now uses 5/4 times more memory for the pending buffer.

Further, a single allocation is now used, which takes additional 56 bytes
for deflate_allocs in 64-bit mode aligned to 16, to store sub-allocation
pointers, and the total allocation size now padded up to 128 bytes, which
takes theoretically 200 additional bytes in total.  This fits though into
"4 * (64 + sizeof(void*))" additional space for ZALLOC used in zlib-ng
2.1.x versions.  The comment was updated to reflect this.

Renaming a temporary file to an empty path ("") returns NGX_ENOPATH
with a subsequent ngx_create_full_path() to create the full path.
This function skips initial bytes as part of path separator lookup,
which causes out of bounds access on short strings.

The fix is to avoid renaming a temporary file to an obviously invalid
path, as well as explicitly forbid such syntax for literal values.

Although Coverity reports about potential type underflow, it is not
actually possible because the terminating '\0' is always included.

Notably, the run-time check is sufficient enough for Win32 as well.
Other short invalid values result either in NGX_ENOENT or NGX_EEXIST
and "MoveFile() .. failed" critical log messages, which involves a
separate error handling.

Prodded by Coverity (CID 1605485).

This simplifies merging protocol values after ea15896 and ebd18ec.

Further, as outlined in ebd18ec18, for libraries preceeding TLSv1.2+
support, only meaningful versions TLSv1 and TLSv1.1 are set by default.

While here, fixed indentation.

When cropping stsc atom, it's assumed that chunk index is never 0.
Based on this assumption, start_chunk and end_chunk are calculated
by subtracting 1 from it.  If chunk index is zero, start_chunk or
end_chunk may underflow, which will later trigger
"start/end time is out mp4 stco chunks" error.  The change adds an
explicit check for zero chunk index to avoid underflow and report
a proper error.

Zero chunk index is explicitly banned in ISO/IEC 14496-12, 8.7.4
Sample To Chunk Box.  It's also implicitly banned in QuickTime File
Format specification.  Description of chunk offset table references
"Chunk 1" as the first table element.