Open a file
Open flags (with the exception of O_CREAT, O_EXCL, O_NOCTTY and O_TRUNC) are available in fi->flags.
Filesystem may store an arbitrary file handle (pointer, index, etc) in fi->fh, and use this in other all other file operations (read, write, flush, release, fsync).
Filesystem may also implement stateless file I/O and not store anything in fi->fh.
Valid replies: fuse_reply_open fuse_reply_err