File size checks fails and I haven't figured out why, but next I'd look into how `posix_fallocate` works on alpine linux.

Log:

```
$ cat /etc/alpine-release 
3.9.2

$ git log --oneline -1
e45b7f20e Merge #15618: refactor: Remove unused function

$ ./src/test/test_bitcoin -t flatfile_tests
Running 4 test cases...
Version using posix_fallocate
test/flatfile_tests.cpp(94): error: in "flatfile_tests/flatfile_allocate": check fs::file_size(seq.FileName(FlatFilePos(0, 0))) == 100 has failed [0 != 100]
test/flatfile_tests.cpp(98): error: in "flatfile_tests/flatfile_allocate": check fs::file_size(seq.FileName(FlatFilePos(0, 99))) == 100 has failed [0 != 100]
Version using posix_fallocate
test/flatfile_tests.cpp(102): error: in "flatfile_tests/flatfile_allocate": check fs::file_size(seq.FileName(FlatFilePos(0, 99))) == 200 has failed [0 != 200]
Version using posix_fallocate
test/flatfile_tests.cpp(116): error: in "flatfile_tests/flatfile_flush": check fs::file_size(seq.FileName(FlatFilePos(0, 1))) == 100 has failed [0 != 100]

*** 4 failures are detected in the test module "Bitcoin Core Test Suite"
```
{
    size_t option_index = key.find('.');
    std::string arg_no_net;
    if (option_index == std::string::npos) {
        arg_no_net = key;
    } else {
        arg_no_net = std::string("-") + key.substr(option_index + 1, std::string::npos);
    }
    LOCK(cs_args);
    for (const auto& arg_map : m_available_args) {
        if (arg_map.second.count(arg_no_net)) return true;
    }
    return false;
}
std::vector<std::string> ArgsManager::GetArgs(const std::string& strArg) const
{
    std::vector<std::string> result = {};
    if (IsArgNegated(strArg)) return result; 
    LOCK(cs_args);
    ArgsManagerHelper::AddArgs(result, m_override_args, strArg);
    if (!m_network.empty()) {
        ArgsManagerHelper::AddArgs(result, m_config_args, ArgsManagerHelper::NetworkArg(*this, strArg));
    }
    if (ArgsManagerHelper::UseDefaultSection(*this, strArg)) {
        ArgsManagerHelper::AddArgs(result, m_config_args, strArg);
    }
    return result;
}
bool ArgsManager::IsArgSet(const std::string& strArg) const