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"
```
const std::string section = str.substr(1, str.size() - 2);
                sections.emplace_back(SectionInfo{section, filepath, linenr});
                prefix = section + '.';
            } else if (*str.begin() == '-') {
                error = strprintf("parse error on line %i: %s, options in configuration file must be specified without leading -", linenr, str);
                return false;
            } else if ((pos = str.find('=')) != std::string::npos) {
                std::string name = prefix + TrimString(str.substr(0, pos), pattern);
                std::string value = TrimString(str.substr(pos + 1), pattern);
                if (used_hash && name.find("rpcpassword") != std::string::npos) {
                    error = strprintf("parse error on line %i, using # in rpcpassword can be ambiguous and should be avoided", linenr);
                    return false;
                }
                options.emplace_back(name, value);
                if ((pos = name.rfind('.')) != std::string::npos && prefix.length() <= pos) {
                    sections.emplace_back(SectionInfo{name.substr(0, pos), filepath, linenr});
                }
            } else {
                error = strprintf("parse error on line %i: %s", linenr, str);
                if (str.size() >= 2 && str.substr(0, 2) == "no") {
                    error += strprintf(", if you intended to specify a negated option, use %s=1 instead", str);
                }
                return false;
            }
        }
        ++linenr;
    }
    return true;
}
bool ArgsManager::ReadConfigStream(std::istream& stream, const std::string& filepath, std::string& error, bool ignore_invalid_keys)