- Replaced QNAP package icons (PNGs support alpha-blending and look better).

- fixed most of the found problems with multithreading
  - noticeable improvements in download speed - it became more stable
  - the download speed dropping to 0 MB/s has gone away
  - the application became more stable, less crashes
- resolved some compiler warnings

Without this, the build fails with:
```
/var/tmp/portage/net-nntp/nzbget-24.1/work/nzbget-24.1/lib/regex/regex.c: In function ‘set_regs’:
/var/tmp/portage/net-nntp/nzbget-24.1/work/nzbget-24.1/lib/regex/regex.c:7701:39: error: implicit declaration of function ‘alloca’; did you mean ‘calloc’? [-Wimplicit-function-declaration]                        7701 |         prev_idx_match = (regmatch_t*)alloca(nmatch * sizeof(regmatch_t));
      |                                       ^~~~~~
      |                                       calloc
```

- fixed the FileSystem tests build on POSIX

- removed the use of Boost.Variant and Boost.Optional since they are no longer relevant after moving to C++17 and GCC 9+.

- fixed speed value overflows if the download speed is greater than 2 GB/s;
- the `Status` xml-rpc response no has the following properties: `DownloadRateLo`, `DownloadRateHi`, `AverageDownloadRateLo`, `AverageDownloadRateHi`. `DownloadRate` and `AverageDownloadRate` are deprecated now.

- fixed int32 overflow;
- improved Util::SafeIntCast to work with unsigned integers;
- added more unit tests.

- according to xml-rpc doc the 'double' type must be used for floating-point numbers.

- added a function to the nzbfile class to derive unpack password from the filename if it is encoded in {{Brackets}}.

- clarified that custom sections require a minimum version of NZBGet 24
- cleaned up the python examples

- added a new "STATUS" page that displays information about the user's environment and hardware and a server speed tests;
- added new API-methods:
  - struct sysinfo() - returns information about the user's environment and hardware;     
  - bool testserverspeed(url, serverId) - puts nzb file to be downloaded by the target server.
  
- API-method "status" now has 3 extra fields:
  - TotalDiskSpaceLo - Total disk space on ‘DestDir’, in bytes. This field contains the low 32-bits of 64-bit value
  - TotalDiskSpaceHi - Total disk space on ‘DestDir’, in bytes. This field contains the high 32-bits of 64-bit value
  - TotalDiskSpaceMB - Total disk space on ‘DestDir’, in megabytes.`
- fixed NZB generator: the last segment was incorrect.
  
## Lib changes

- Boost.Asio - cross-platform library for network.

- added nzbget installation without nzbget.conf;
- strip the binary in CMake.

- added v24.2 changelog

The commit c5dce75 causes the build to fail on OpenBSD due to the use of
the `sysctlbyname` function, which is not available on OpenBSD. This
issue can be resolved by replacing `sysctlbyname` with the `sysctl`
function, which is supported across all BSD variants.

When building with `-DCMAKE_BUILD_TYPE=Debug`, using clang, the wrong
`CMAKE_C_FLAGS_DEBUG` are set, which causes a wrong debug build. This
issue is caused by 0043452, which sets `CMAKE_CXX_FLAGS_DEBUG` to the
correct flags, but later is set to `CMAKE_CXX_FLAGS`, which is empty.

Issue does not occur when building with a different compiler.

The commit c5dce75 introduces a build failure on systems using LibreSSL.
LibreSSL, a fork of OpenSSL, aims to modernize the codebase, enhance
security, and follow best development practices. The build failure is
due to the replacement of `OPENSSL_FULL_VERSION_STR` with
`LIBRESSL_VERSION_TEXT`. The latter requires some string manipulation
since it is defined as "LibreSSL major.minor.patch".