gvfsd-fuse: support optional FUSE flags, make FUSE_CAP_ATOMIC_O_TRUNC one of them
The vast majority of OSes (OSX, FreeBSD, NetBSD, OpenBSD, Illumos) can't support many create()/open() flags.
For non-critical flags, allow falling back to working without them. Treat FUSE_CAP_ATOMIC_O_TRUNC as one of those.
Closes #553