This commit contains the following changes:
- Rewrite vfile close/read/write packet handlers with portable routines from lldb. This removes #if(s) and allows the handlers to work on Windows.
- Fix a bug in File::Write. This is intended to write data at an offset to a file but actually writes at the current position of the file.
- Add a default boolean argument 'should_close_fd' to FileSystem::Open to let the user decide whether to close the fd or not.