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"
```
}
        
        if (arg_map.first == OptionsCategory::HIDDEN) break;
        for (const auto& arg : arg_map.second) {
            if (show_debug || !arg.second.m_debug_only) {
                std::string name;
                if (arg.second.m_help_param.empty()) {
                    name = arg.first;
                } else {
                    name = arg.first + arg.second.m_help_param;
                }
                usage += HelpMessageOpt(name, arg.second.m_help_text);
            }
        }
    }
    return usage;
}
bool HelpRequested(const ArgsManager& args)
{
    return args.IsArgSet("-?") || args.IsArgSet("-h") || args.IsArgSet("-help") || args.IsArgSet("-help-debug");
}
void SetupHelpOptions(ArgsManager& args)
{
    args.AddArg("-?", "Print this help message and exit", false, OptionsCategory::OPTIONS);
    args.AddHiddenArgs({"-h", "-help"});
}
static const int screenWidth = 79;
static const int optIndent = 2;
static const int msgIndent = 7;
std::string HelpMessageGroup(const std::string &message) {