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"
```
break;
            case OptionsCategory::NODE_RELAY:
                usage += HelpMessageGroup("Node relay options:");
                break;
            case OptionsCategory::BLOCK_CREATION:
                usage += HelpMessageGroup("Block creation options:");
                break;
            case OptionsCategory::RPC:
                usage += HelpMessageGroup("RPC server options:");
                break;
            case OptionsCategory::WALLET:
                usage += HelpMessageGroup("Wallet options:");
                break;
            case OptionsCategory::WALLET_DEBUG_TEST:
                if (show_debug) usage += HelpMessageGroup("Wallet debugging/testing options:");
                break;
            case OptionsCategory::CHAINPARAMS:
                usage += HelpMessageGroup("Chain selection options:");
                break;
            case OptionsCategory::GUI:
                usage += HelpMessageGroup("UI Options:");
                break;
            case OptionsCategory::COMMANDS:
                usage += HelpMessageGroup("Commands:");
                break;
            case OptionsCategory::REGISTER_COMMANDS:
                usage += HelpMessageGroup("Register Commands:");
                break;
            default:
                break;