assemble multidevice btrfs volumes without external tools
This self-contained approach introduce very little overhead, unless
someone has a large number of devices composing many btrfs volumes,
in which case btrfs device scan would be faster. Still, having robust
implementation is a nice to have alternative for btrfs-progs.
All "btrfs" file systems will be registered with the kernel when they
show up.
Incomplete multi-device volumes will set SYSTEMD_READY=0, to prevent
access until the volume is complete and fully registered.