Working on "Use std::filesystem. Remove Boost Filesystem & System" #20744, I found an unfortunate [pitfall](https://github.com/bitcoin/bitcoin/pull/20744#issuecomment-946046666) -- `boost/process.hpp` depends on `boost::filesystem` when cross-compiling for Windows. It happens in Guix environment as well.

The goal of this issue is to discuss and find a solution to get rid of `boost::filesystem`.
{
    std::string datadir = gArgs.GetArg("-datadir", "");
    return datadir.empty() || fs::is_directory(fs::system_complete(fs::PathFromString(datadir)));
}
fs::path GetConfigFile(const std::string& confPath)
{
    return AbsPathForConfigVal(fs::PathFromString(confPath), false);
}
static bool GetConfigOptions(std::istream& stream, const std::string& filepath, std::string& error, std::vector<std::pair<std::string, std::string>>& options, std::list<SectionInfo>& sections)
{
    std::string str, prefix;
    std::string::size_type pos;
    int linenr = 1;
    while (std::getline(stream, str)) {
        bool used_hash = false;
        if ((pos = str.find('#')) != std::string::npos) {
            str = str.substr(0, pos);
            used_hash = true;
        }
        const static std::string pattern = " \t\r\n";
        str = TrimString(str, pattern);
        if (!str.empty()) {
            if (*str.begin() == '[' && *str.rbegin() == ']') {