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"
```
#include <sys/stat.h>
#else
#ifdef _MSC_VER
#pragma warning(disable:4786)
#pragma warning(disable:4804)
#pragma warning(disable:4805)
#pragma warning(disable:4717)
#endif
#ifdef _WIN32_IE
#undef _WIN32_IE
#endif
#define _WIN32_IE 0x0501
#define WIN32_LEAN_AND_MEAN 1
#ifndef NOMINMAX
#define NOMINMAX
#endif
#include <codecvt>
#include <io.h> 
#include <shellapi.h>
#include <shlobj.h>
#endif
#ifdef HAVE_SYS_PRCTL_H
#include <sys/prctl.h>
#endif
#ifdef HAVE_MALLOPT_ARENA_MAX
#include <malloc.h>
#endif
#include <thread>
const int64_t nStartupTime = GetTime();
const char * const BITCOIN_CONF_FILENAME = "bitcoin.conf";