Add "--proto 0" to send/receive

[madsl]

Send/receive has received support for a new send protocol in kernel 5.19. Btrfs-sxbackup should default to the newest protocol supported given that the kernel version and btrfs-progs version supports it.

Using "btrfs send --proto 0" should be a safe way to do it.

[masc3d]

doesn't this break backups running older kernels. imho btrfs send / receive should negotiate their protocol version and until that happens its probably better sticking with default. what's the concrete benefits of running newest protocol via --proto 0?

[madsl]

       --proto <N>
              use send protocol version N

              The default is 1, which was the original protocol version. Version 2 encodes file data slightly more efficiently; it is also required for sending compressed data directly
              (see  --compressed-data).  Version  2 requires at least btrfs-progs 5.18 on both the sender and receiver and at least Linux 5.18 on the sender. **Passing 0 means to use the
              highest version supported by the running kernel.**

So if I understand this correctly, I think proto 0 means it finds out itself what is safe.