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 (strCommand.empty()) return;
#ifndef WIN32
    int nErr = ::system(strCommand.c_str());
#else
    int nErr = ::_wsystem(std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>,wchar_t>().from_bytes(strCommand).c_str());
#endif
    if (nErr)
        LogPrintf("runCommand error: system(%s) returned %d\n", strCommand, nErr);
}
void RenameThread(const char* name)
{
#if defined(PR_SET_NAME)
    
    ::prctl(PR_SET_NAME, name, 0, 0, 0);
#elif (defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__DragonFly__))
    pthread_set_name_np(pthread_self(), name);
#elif defined(MAC_OSX)
    pthread_setname_np(name);
#else
    
    (void)name;
#endif
}
void SetupEnvironment()