Handle certificate chain generation correctly.

Thanks to @orisano .
a bug fixed that the module sometimes returns invalid corrupted cert+cbor file.

Add sxg_fallback_host directive

Now we can manually modify fallback-url embedded into SXG file with sxg_fallback_host directive in nginx config file.

Return HTTP 200 OK status for successful SXG.

Merged 2 patches
#83 : Now this module returns HTTP 200 status code with generated SXG.
#86 : New .deb files for Ubuntu Groovy Gorilla (20.10) instead of Eoan Ermine (19.10).

Embedded fallback URL is now requested URL

Merged #78
An SXG file has a fallback URL to be used when something wrong with the SXG file. But in previous implementation, it is modified to the actual URL and for example an access to https://example.com/ returns SXG embedding https://example.com/index.html.
It turned out to be an problematic behavior when user expect the fallback URL matches queried URL especially in the context of subresource fetching.
From now, request to https://example.com/ will return SXG with embedding URL https://example.com/ even though the actual contents exists in index.html.

Configurable life-span of generated SXG

New feature

• Newly appended sxg_expiry_seconds directive in nginx configuration, which enables uses to modify the life-span of generated SXG files (referenced as expires parameter in the SXG header`).

Configure as location

Configuration

In this release, we normalized on recognizing nginx configuration.
Previously, we cannot use more than one sever directive in whole nginx configuration, it was a bug.
Now this is improved and adopts multiple server and location directives.
You can write sxg directive both in server and location directives, so you can switch ON or OFF for multiple location in nginx configuration.
Notice that you cannot specify sxg_certificate and other directives in location, it is only allowed in server scope, this is because one server should not have multiple SXG certificates (and keys).
This fix is done by @orisano in #66 . Thank you!

Headers

Now nginx-sxg-module will not return Etag or other upstream header to the user as a outer header. These headers will be packed into inner SXG header.
And it appends X-Content-Type-Options: nosniff as outer header automatically. This header is required by SXG spec and previously users should append this header via nginx directives. If you already appended the header entry by your hands, the header with this version will become like X-Content-Type-Options: nosniff,nosniff. This is not harmful but we recommend to remove your appended header.

And other small fixes are applied, enjoy SXG!

Improved stability

• When preload header contains full path URL, this module just ignore that. (previously it caused hang up)
• Appended mutex to guard the request context against concurrently updated by multiple subresources.

Minor fix

Eliminated some compiler warnings.
Nothing changed from v3.4.0 in binary level (I believe).

Bug fix

This release includes several important bugfixes.

• Drop link: </foo>rel="preload" of failed to fetch subresources.
• Defeated some mysterious bugs which stop the module and cause client timeout by correction of handling buffer. (Thanks to @orisano for #38)
• Accept HTTP Link headers with case insensitive (e.g. LiNk: ;rel="preload">. (Thanks to @sisidovski for #41)
• Embed URL parameter into SXG fallback URL (#36).
• Some debug error logs appended only appears when you use it like error_log path/to/error.log debug.

Improve error messages.

In this version, nginx-sxg plugin will provide a bit more verbose error messages for failure on starting up.