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"
```
{
    std::set<std::string> unsuitables;
    LOCK(cs_args);
    
    if (m_network.empty()) return std::set<std::string> {};
    
    if (m_network == CBaseChainParams::MAIN) return std::set<std::string> {};
    for (const auto& arg : m_network_only_args) {
        std::pair<bool, std::string> found_result;
        
        found_result = ArgsManagerHelper::GetArgHelper(m_override_args, arg);
        if (found_result.first) continue;
        
        found_result = ArgsManagerHelper::GetArgHelper(m_config_args, ArgsManagerHelper::NetworkArg(*this, arg));
        if (found_result.first) continue;
        
        found_result = ArgsManagerHelper::GetArgHelper(m_config_args, arg);
        if (!found_result.first) continue;
        
        unsuitables.insert(arg);
    }
    return unsuitables;
}
const std::list<SectionInfo> ArgsManager::GetUnrecognizedSections() const