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"
```
/* Special test for -testnet and -regtest args, because we
     * don't want to be confused by craziness like "[regtest] testnet=1"
     */
    static inline bool GetNetBoolArg(const ArgsManager &am, const std::string& net_arg) EXCLUSIVE_LOCKS_REQUIRED(am.cs_args)
    {
        std::pair<bool,std::string> found_result(false,std::string());
        found_result = GetArgHelper(am.m_override_args, net_arg, true);
        if (!found_result.first) {
            found_result = GetArgHelper(am.m_config_args, net_arg, true);
            if (!found_result.first) {
                return false; 
            }
        }
        return InterpretBool(found_result.second); 
    }
};
/**
 * Interpret -nofoo as if the user supplied -foo=0.
 *
 * This method also tracks when the -no form was supplied, and if so,
 * checks whether there was a double-negative (-nofoo=0 -> -foo=1).
 *
 * If there was not a double negative, it removes the "no" from the key,
 * and returns true, indicating the caller should clear the args vector
 * to indicate a negated option.
 *
 * If there was a double negative, it removes "no" from the key, sets the
 * value to "1" and returns false.
 *
 * If there was no "no", it leaves key and value untouched and returns