Issue: #4634

* [CritFix] Fix leak in `gzip` function
* [Feature] Add ICAP Content-Type and Filename
* [Feature] Add `logging`->`task_max_elts` option
* [Feature] Add utility to split string like stuff for C++ code
* [Feature] Allow to set HTTP auth parameters for the maps
* [Feature] Check for plugin configuration errors on `configtest`
* [Feature] `known_senders` plugin
* [Feature] Use backward-cpp instead of manual libunwind stuff
* [Feature] rbl: support checking numeric URLs in isolation
* [Fix] CMakeLists.txt remove whitespace added by linter as it makes tests fail
* [Fix] Change Date: header location to conform with RFC
* [Fix] Correct format pattern for RE tree tempfile name
* [Fix] Correct format string for unw_word_t
* [Fix] Do not accept invalid ucl object types
* [Fix] Do not pollute public headers with libev internals
* [Fix] Do not set output type if list application failed
* [Fix] Fix `url:set_redirected` method
* [Fix] Fix format string and some length issues
* [Fix] Fix grammar definition for content-disposition attributes
* [Fix] Fix lua schema enrichment logic for Redis params
* [Fix] Fix lua stack corruption when logging large tables
* [Fix] Fix merge table utility
* [Fix] Fix output of non-RSA DKIM keys
* [Fix] Fix some corner cases of single-host urls parsing
* [Fix] Fix various issues in the `url_redirector` plugin
* [Fix] MISSING_MIMEOLE: avoid matching messages from Android GMail app (#4561)
* [Fix] Prevent DNSWL sabotage
* [Fix] Try to fix unzip function
* [Fix] rbl: really fix dependency registration when symbols_prefixes is used
* [Fix] rspamadm mime: arguments beginning with letter `t`
* [Rework] Breaking: return back to semver
* [Rework] Move rcl logic to C++

A space seems to have sneaked into the setting of LUA_PATH environment.

Issue: #4552

rspamd uses G_(BEGIN|END)_DECLS not __(BEGIN|END)_DECLS

Default pkg-config name of zstd is libzstd.pc. Let's add it to the
MODULES list.

It is now possible to use system xxhash since commits 23889da9ae
("[Minor] Update bundled xxhash") and 7ad02fc1f7 ("[Rework] Use xxh3
as a default hash and fix memory/alignment issues"). Let's give
packagers this possibility with a new SYSTEM_XXHASH option.

[Rework] Massive spelling fix from @jsoref

There are 2 major versions of the PCRE library, PCRE and PCRE2. Rspamd
supports both and current default is the former. But according to the
web page of PCRE library it has reached its end of life. So change the
default to the latter.

This gives packagers option to use system version of doctest rather than
bundled one. It is disabled by default. Additionally, there is no need
to link rspamd-test-cxx with doctest as it is is single header library.

Main reason for this change is that currently bundled version of
doctest-2.4.5 can't compile with glibc-2.34 [1]. However, this issue was
already fixed in upstream doctest-2.4.6 [2] in commit [3]. With this,
packagers can just bump doctest to fixed version.

[1] https://github.com/onqtam/doctest/issues/473
[2] https://github.com/onqtam/doctest/blob/master/CHANGELOG.md#246-2021-03-22
[3] 099d5414e9

This gives packagers option to use system version of fmt rather than
bundled one. It is disabled by default.

* fixes #3826

* fixes #3807