No way to create empty + appendable files via file_descriptor c-tor

This is perhaps some corner case, but consider that you want to use `file_descriptor` to manage some log file. Writes to log files are typically write-only-to-the-end, but you may also want to do random reads on logs. Random reads via `pread` are no problem in Linux (they don't move file pointer), but random reads via `ReadFile` with offsets specified in `OVERLAPPED` struct actually move file pointer, so subsequent writes will not be at the end of file.

The problem is fixed by opening a file with O_APPEND/FILE_APPEND_DATA when creating a new file. It would be nice to be able to forcefully open a "fresh" file with `std::ios_base::app flag`. I imagine this could be done by allowing `std::ios_base::trunc | std::ios_base::app` combo. The effect should be as follows:

```C++
#if defined(BOOST_IOSTREAMS_WINDOWS)
DWORD dwDesiredAccess;
if (enable_append) {
dwDesiredAccess = GENERIC_READ | FILE_APPEND_DATA | FILE_WRITE_ATTRIBUTES | FILE_WRITE_EA |
STANDARD_RIGHTS_WRITE | SYNCHRONIZE;
} else {
dwDesiredAccess = GENERIC_READ | GENERIC_WRITE;
}

HANDLE file_handle =
::CreateFileA(file_name.c_str(), dwDesiredAccess, FILE_SHARE_READ | FILE_SHARE_WRITE,
NULL, // lpSecurityAttributes
CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL,
NULL); // hTemplateFile
#else
// Calculate oflag argument to open.
int oflag = O_CREAT | O_TRUNC | O_RDWR;
if (enable_append) {
oflag |= O_APPEND;
}
#ifdef _LARGEFILE64_SOURCE
oflag |= O_LARGEFILE;
#endif

// Calculate pmode argument to open.
mode_t pmode = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH;

// Open file.
int file_handle = BOOST_IOSTREAMS_FD_OPEN(file_name.c_str(), oflag, pmode);
#endif
```

Start at the file_descriptor constructor and its platform-specific file-opening logic. Compare the requested std::ios_base::trunc | std::ios_base::app behavior with the Windows CreateFileA and POSIX open flags shown in the issue. Done means a newly created file is empty, supports reads, and appends writes after random reads on both platforms.

Written by the indexing model from the issue text.