Support OFD locks?

[andrewchambers]

It seems linux now supports private file locks: https://www.gnu.org/software/libc/manual/html_node/Open-File-Description-Locks.html

These essentially solve the large problem diod has implementing POSIX locks (The fact they are per pid which is essentially impossible to implement) - it seems like we would need to extend the protocol to add these new lock types and teach the v9fs driver about them.

I think adding the new flags would be backwards compatible, old servers and clients would likely just return an error.