This patch removes includes that are not used. The removals were found with
include-what-you-use which checks if any of the symbols from a header is
in use.
include-what-you-use automatically does this and it makes finding
unnecessary harder to spot. The only content of poll.h is a include
of sys/poll.h so should be harmless.
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.