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"
```
bool emptyIncludeConf;
        {
            LOCK(cs_args);
            emptyIncludeConf = m_override_args.count("-includeconf") == 0;
        }
        if (emptyIncludeConf) {
            std::string chain_id = GetChainName();
            std::vector<std::string> includeconf(GetArgs("-includeconf"));
            {
                
                
                std::vector<std::string> includeconf_net(GetArgs(std::string("-") + chain_id + ".includeconf"));
                includeconf.insert(includeconf.end(), includeconf_net.begin(), includeconf_net.end());
            }
            
            
            {
                LOCK(cs_args);
                m_config_args.erase("-includeconf");
                m_config_args.erase(std::string("-") + chain_id + ".includeconf");
            }
            for (const std::string& to_include : includeconf) {
                fsbridge::ifstream include_config(GetConfigFile(to_include));
                if (include_config.good()) {
                    if (!ReadConfigStream(include_config, to_include, error, ignore_invalid_keys)) {
                        return false;
                    }
                    LogPrintf("Included configuration file %s\n", to_include.c_str());
                } else {