Updated Continuous Integration

[tdulcet]

• Updated the Continuous Integration (CI) from #217.
  • Changed GitHub Actions to also build with Ubuntu 22.04.
  • Removed Ubuntu 18.04, which never worked.
  • Changed GitHub Actions to also build with Clang on Windows.
  • Set Cppcheck to check all #ifdef configurations.
  • Configured GitHub Actions so users can download the resulting GpuOwl binaries.
  • This is mainly for Windows and other users who are unable to directly build GpuOwl. They can be downloaded from the "Actions" tab above.
• Updated help output to get default values from the respective variables.
• Updated the README.
  • Updated command line options section to reflect the current gpuowl -h output.
  • Minor copy edits.

Note that https://github.com/preda/gpuowl/commit/817a9831dc9fdc595709bd5f68482e654b758b65 broke Clang support, so the CI currently allows those builds to fail. I would be happy to remove these builds and update the README if Clang is no longer supported.