…urity v3

- All source code files need to include `ngx_config.h` and this needs
  to be the first header file included.

- Added define to avoid the following warning, treated as an error
  by the nginx build system:
  warning C4996: 'strdup': The POSIX name for this item is deprecated. Ins
tead, use the ISO C++ conformant name: _strdup. See online help for details.

- `ngx_http_modsecurity_process_intervention` &
  `ngx_http_modsecurity_create_ctx` are exported by
  `ngx_http_modsecurity_module.c`, so they can't be declared as
  inline because the MSVC compiler will not export them in the
  generated object file.

- Windows Docker container to build libModSecurity v3 & nginx w/ModSecurity-nginx

- Include steps to run all ModSecurity-nginx tests
- Simplify checkout of libModSecurity