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"
```
return std::string(message) + std::string("\n\n");
}
std::string HelpMessageOpt(const std::string &option, const std::string &message) {
    return std::string(optIndent,' ') + std::string(option) +
           std::string("\n") + std::string(msgIndent,' ') +
           FormatParagraph(message, screenWidth - msgIndent, msgIndent) +
           std::string("\n\n");
}
static std::string FormatException(const std::exception* pex, const char* pszThread)
{
#ifdef WIN32
    char pszModule[MAX_PATH] = "";
    GetModuleFileNameA(nullptr, pszModule, sizeof(pszModule));
#else
    const char* pszModule = "bitcoin";
#endif
    if (pex)
        return strprintf(
            "EXCEPTION: %s       \n%s       \n%s in %s       \n", typeid(*pex).name(), pex->what(), pszModule, pszThread);
    else
        return strprintf(
            "UNKNOWN EXCEPTION       \n%s in %s       \n", pszModule, pszThread);
}
void PrintExceptionContinue(const std::exception* pex, const char* pszThread)
{
    std::string message = FormatException(pex, pszThread);
    LogPrintf("\n\n************************\n%s\n", message);
    fprintf(stderr, "\n\n************************\n%s\n", message.c_str());
}
fs::path GetDefaultDataDir()