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"
```
public:
    typedef std::map<std::string, std::vector<std::string>> MapArgs;
    /** Determine whether to use config settings in the default section,
     *  See also comments around ArgsManager::ArgsManager() below. */
    static inline bool UseDefaultSection(const ArgsManager& am, const std::string& arg) EXCLUSIVE_LOCKS_REQUIRED(am.cs_args)
    {
        return (am.m_network == CBaseChainParams::MAIN || am.m_network_only_args.count(arg) == 0);
    }
    
    static inline std::string NetworkArg(const ArgsManager& am, const std::string& arg)
    {
        assert(arg.length() > 1 && arg[0] == '-');
        return "-" + am.m_network + "." + arg.substr(1);
    }
    
    static inline void AddArgs(std::vector<std::string>& res, const MapArgs& map_args, const std::string& arg)