Long case insensitive style also makes short case insensitive.

When you use the *::long_case_insensitive* style it inadvertently behaves as if you also selected *::short_case_insensitive*.

I noticed this behavior since boost 1.68.0, compiling with 1.67.0 gives the correct result.

In v1.67.0, if you call the test program below like this, `./test -A`, it will give the correct result:

```
Adam is selected: false
Alice is selected: true
```

In v1.68.0 and v1.69.0 it will give the wrong result, as if *::short_case_insensitive* was also selected.:

```
Adam is selected: true
Alice is selected: false
```

Basically the *-A* argument becomes an *-a* argument.

## Example code to illustrate the problem.

```
#include
#include

namespace po = boost::program_options;
namespace po_style = boost::program_options::command_line_style;

int main(int argc, char* argv[]){

bool adamSelected = false;
bool aliceSelected = false;

try{
po::options_description desc("Allowed options");
desc.add_options()
("adam,a", po::value(&adamSelected)->default_value(false)->implicit_value(true),"Choose Adam")
("alice,A", po::value(&aliceSelected)->default_value(false)->implicit_value(true),"Choose Alice")
;

po::variables_map vm;

auto parsed = po::command_line_parser(argc, argv).options(desc).style(po_style::unix_style | po_style::long_case_insensitive).run();
po::store(parsed, vm);

po::notify(vm);

std::cout << "Adam is selected: " << (adamSelected?"true":"false") << std::endl
<< "Alice is selected: " << (aliceSelected?"true":"false") << std::endl;
}
catch(std::exception& e){
std::cerr << e.what() << std::endl;
}
return 0;
}

```

提供された command_line_parser の例、特に ./test -A を使って、まず問題を再現し、コマンドラインスタイルの解析パスで long_case_insensitive がどのように扱われているかを追跡します。long_case_insensitive によって短いオプションが大文字と小文字を区別しなくなることがなくなり、1.67.0 の期待される出力が維持され、この動作を対象とするリグレッションテストが追加されれば完了です。

索引モデルが issue の本文から書いたものです。