`subvol sync`: add implementation using SUBVOL_SYNC_WAIT ioctl

[kdave]

Kernel 6.13 provides a new unprivileged ioctl to wait for subvolume cleaning (https://git.kernel.org/linus/6c83d153ed86eb17c46eafe4e78af4ce2071a05 "btrfs: add new ioctl to wait for cleaned subvolumes")

• try the new ioctl first, if it does not exist fall back to the old SEARCH_TREE way
• the new ioctl is not 1:1 functionality and does not know the full list of subvoulmes being cleaned; add an option to force the SEARCH_TREE ioctl that can do that