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"
```
{
    return AbsPathForConfigVal(fs::path(confPath), false);
}
static std::string TrimString(const std::string& str, const std::string& pattern)
{
    std::string::size_type front = str.find_first_not_of(pattern);
    if (front == std::string::npos) {
        return std::string();
    }
    std::string::size_type end = str.find_last_not_of(pattern);
    return str.substr(front, end - front + 1);
}
static bool GetConfigOptions(std::istream& stream, const std::string& filepath, std::string& error, std::vector<std::pair<std::string, std::string>>& options, std::list<SectionInfo>& sections)
{
    std::string str, prefix;
    std::string::size_type pos;
    int linenr = 1;
    while (std::getline(stream, str)) {
        bool used_hash = false;
        if ((pos = str.find('#')) != std::string::npos) {
            str = str.substr(0, pos);
            used_hash = true;
        }
        const static std::string pattern = " \t\r\n";
        str = TrimString(str, pattern);
        if (!str.empty()) {
            if (*str.begin() == '[' && *str.rbegin() == ']') {